To add on Evil's list. Here is:
Fly's Official Low FPS Console Command Collection
Discovered and Assembled by Glowth (so i can't actually take credit for it)
Recommended launch options:
-console -high -threads # +cl_cmdrate 67 +cl_updaterate 67 +rate 30000
-console Enables console
-high Set's Gmod's processing priority higher than normal, which can keep background applications from lagging your game. However, note that it can potentially cause issues with things like Steam overlay, Skype, or other VOIP programs, ecspecially on dual core machines.
-threads <#> Google your CPU specs and see how many threads you have. I have an i5-4690k, so I have 4 threads: -threads 4
+cl_cmdrate 67
+cl_updaterate 67
+rate 30000
These won't impact performance, it will help your connection to servers, potentially making things like props move smoother. Unlikely but might help.
-winxp Runs the game in windows compatibility mode. Possible fps boost? Might crash upon Alt Tabbing the game on lower end pc's. Please comment below if this command made any difference.
-nod3d9ex Disables Windows Aero Extensions
A somewhat recent graphics card won't see much of a performance boost from setting this, it'll more than likely hurt performance and can cause instability from alt+tabbing.
-directX 9.0/8.0/7.0 - don't exist, there's -dxlevel which will reset your video options every launch, use +mat_dxlevel 95
With a somewhat recent graphics card (GeForce 700 series or later, Radeon HD 6000 series or later), this won't have much if any positive impact on performance. Setting +max_dxlevel 90 may help performance for some however, it'll disable a lot of effects that don't really impact performance on decent PCs (DirectX 9.0 vs. DirectX 9.0c), and can actually cause other issues. +mat_dxlevel 81 and 80 cause many issues
If you put these in your launch options, your game will most likely get an fps boost. Remember that if you really want more fps, check out the autoexec and the commands below.
Ingame console commands
sv_autoladderdismount <0,1> If set to 1, the player automatically dismounts from ladders when the end is reached. If disabled, the player must press the USE key to mount/dismount ladders and hence cannot accidentally jump/walk off a ladder. Set to taste as there is no performance impact.
stopsound If you hear a constant* sound of something, type stopsound in the console.
flush Flushes the cache memory. Can resolve graphical anomalies such as texture glitches???
exit Shuts down Gmod.
quit Shuts down Gmod.
bind <key, command> example: bind g "say Hello!" Will make you say "Hello!" In the chat box.
r_cleardecals Clears all current decals on the map. Note: This only removes them, it does NOT stop new ones from forming, so it's only temporary.
bindtoggle <key, command> Same as the bind command, expect the assigned key can be used to both turn the command on and off
cl_show_splashes 0 Disables water splashes.
r_cheapwaterstart 1 Lower water quality.
r_cheapwaterend 1 Lower water quality.
r_waterforceexpensive 0 Lower water quality.
r_WaterDrawReflection 0 Disables water reflection.
mat_wateroverlaysize 4 Lower water resolution.
r_ForceWaterLeaf 0 No water leaf.
fog_enable_water_fog 0 Removes underwater fog.
mat_drawwater 0 Removes water. (Probably requires sv_cheats)
r_DrawRain 0 Disables visible rain. (Probably requires sv cheats)
mp_decals <#> to limit number of decals. Lower = better performance.
r_decals <#> Max amount of decals possible. Lower = better performance.
r_drawdecals 0 No decals are drawn. (Probably requires sv_cheats)
mp_decals 0 Disables decals.
cl_detail_max_sway 0 No detail sway.
r_drawparticles 0 Disables particle effects. (Probably requires sv_cheats)
r_decal_cullsize <#> Higher = more performance. Decals higher than the specified size in pixels will be removed straight away.
r_drawmodeldecals 0 Decals disabled on models.
r_maxmodeldecal <#> If r_drawmodeldecals is enabled, this number determines the maximum number of decals which can be drawn on a model. Lower = more performance.
r_drawbatchdecals 1 Draw batch decals.
r_shadows 0 Disables shadows and player flashlights.
r_dynamic 0 Disables dynamic lighting.
r_maxdlights <#> Max number of dynamic lights visible on screen if r_dynamic is set to 1
r_flashlightmodels 0 Disables flash light models
r_PhysPropStaticLighting 0 Disables Lighting on static props.
mat_disable_lightwarp 1 Disables lightwarp.
mat_filterlightmaps 0 Disables filterlightmaps.
mat_specular 0 Turns off specular lighting. Less shine.
mat_filterlightmaps 0 Pre-rendered lighting will be blocky and unrealistic.
r_lightaverage 0 Disables light average.
cl_playerspraydisable 1 Disables player sprays.
r_spray_lifetime 1 User preference, how long the spray will be visible. 1=1 round.
M9KGasEffect 0 Removes gun smoke effect from M9K guns while firing.
cl_ejectbrass 0 Weapons will not eject shells while firing.
muzzleflash_light 0 Muzzle flash is still visible, but no lighting is cast from it.
r_drawflecks 0 Removes bullet impact particles.
cl_wpn_sway_interp 0 No weapon sway.
cl_ragdoll_physics_enable 0 Makes player models disappear once they are killed.
r_lod <-8,8> Rate at which level of detail is stripped from objects as they recede away from the player. The lower the value, the more detail remains visible on objects as they progress into the distance, reducing performance but increasing image quality. (Probably requires sv_cheats)
r_rootlod 2 Lower quality on player models.
r_teeth 0 No teeth on player models.
r_eyes 0 No eyes on player models.
lod_enable 1 Enables the LOD system for player models.
lod_transitiondist 400 At wich distance the models should be detailed, 400 = lowest value.
r_drawmodeldecals 0 Disable model decals, for example; blood on the player models.
r_flex 0 // Disabels flex, model related.
ai_expression_optimization 1 Disables NPC expressions when you can't see them.
violence_ablood 0 Disables alien blood when they are hit.
violence_agibs 0 Disables alien gore when they are hit.
violence_hblood 0 Disables human blood when they are hit.
violence_hgibs 0 Disables human gore when they are hit.
cl_phys_props_max 0 Sets range at which props show up.
cl_phys_props_enable 0 Disables little cans/bottles that are rendered client side.
cl_phys_props_max 0 If you break a table the small parts will disapear?
func_break_max_pieces 0 If you break a table the small parts will disapear?
props_break_max_pieces 0 If you break a table the small parts will disapear?
props_break_max_pieces_perframe 0 If you break a table the small parts will disapear?
r_threaded_particles <#> Determines whether the particle system is multi-threaded. This should be set to 1 on systems with multi-core CPUs.
r_threaded_renderables <#> Determines whether part of the rendering system is multi-threaded. This can be set to 1 on systems with multi-core CPUs.
snd_mix_async <#> If set to 1, this command will allow multi-core CPUs to run most of the sound engine in parallel with the renderer, which can improve performance and reduce stuttering.
cl_detaildist <#> Range at which detail props are shown (e.g. grass) Lower = more performance.
cl_detailfade <#> Range at which detail props fade into view. Higher = more performance.
cl_maxrenderable_dist <#> Max render distance from the camera at which things will be rendered. The smaller the number, the less will be rendered. (Probably requires sv_cheats)
lod_TransitionDist <#> Distance at which lod is reduced on objects. Lower = more performance.
sv_robust_explosions 0 Explosions will be less spectacular.
mat_parallaxmap 0 Decreases visual quality.
mat_filtertextures 0 Textures will be blocky and unrealistic.
mat_mipmaptextures 0 Disables minimap textures, which harshens textures.
mat_bumpmap 0 Disables bump mapping which makes flat 2D textures appear three dimensional.
mat_compressedtextures <0,1> - If set to 1, uses compressed textures for optimal performance. For those with graphics cards which have higher amounts of Video RAM (e.g. 256MB or more) setting this to 0 will improve texture appearance at the possible cost of some performance, but may cause a crash.
mat_bloomscale 0 Disables bloom.
mat_disable_bloom 1 Double disable bloom.
mat_antialias 0 Disables Antialiasing.
mat_forceaniso 0 Double Disable Antialiasing.
mat_hdr_enabled 0 Disables High Dynamic Range, HDR.
mat_hdr_level 0 Double Disable High Dynamic Range, HDR.
mat_reducefillrate 1 Reduces fillrate, sets the in-game option Shader detail to low. Should be set to 1 if using dxlevel 80 or 81.
mat_disable_fancy_blending 1 Disables fancy blending.
mem_force_flush 1 Forces the cache to be flushed on every allocation, which is not necessarily optimal nor recommended, but can reduce memory-related errors and crashes.
cl_autohelp 0 Disables some annoying pop-ups.
cl_forcepreload 1 Forces all information to preload.
sv_forcepreload 1 Forces server-side preloading.
cl_smooth 0 Disable smoothing. Reduces stuttering.
fog_enable 0 Will disable fog, which can improve/reduce performance depending on your GPU.
r_3dsky 0 Removes all the background outside of the map.
r_drawskybox 0 Sky textures completely removed. Visual anomalies in the sky.
r_drawtranslucentrenderables 0 All translucent objects will not be drawn. (Combine energy shields, etc. Probably requires sv_cheats)
r_renderoverlayfragment 0 This will remove text on the walls and paper etc.
rope_smooth 0 Disables smoothing on ropes.
rope_wind_dist 0 Ropes will not move?
rope_shake 0 Ropes will not shake.
r_drawropes 0 Disables rope drawing. (Probably requires sv_cheats)