Tutorials on basic VRChat gameplay/controls:
You can adjust avatar safety settings to hide some features or even entire avatars if they're below a certain optimization threshold.
The same can be done for dynamic bone limits.
Both of these will have a noticeable visual impact, but can help significantly depending on how well optimized nearby avatars are.
Launch VRChat in non-VR mode while holding Shift to open up an options menu.
This will allow you to set Screen Resolution and Graphics Quality.
Turning down Screen Resolution will lower the resolution of the desktop mirror window and the outlines of selected objects.
Turning down Graphics Quality will lower Anti-Aliasing (and some other stuff I can't remember)
To set the screen resolution lower than the settings menu allows, you can add launch parameters in Steam.
Right click VRChat in your game list.
Go to Properties.
Click Set Launch Options.
Add "-screen-width 360 -screen-height 240" or whatever your desired resolution is.
If you're using an Oculus headset, you can run the Oculus Store version for less overhead.
You can also change the Oculus dashboard compatibility to "Run as Administrator", which prevents its UI from taking up even more of your precious CPU time.
Overclock your CPU if possible, VRChat doesn't currently utilize multithreading well, so raw single core performance helps the most with framerate.
You can run VRChat directly from the exe in the install without Steam running if you're desperate for every frame you can get.
TTS + VC Guide:
starting point would be UIExpansionKit & VRChatUtilityKit
as for useful mods No outlines Portable Mirrors Avatar Limbs Grabber & AvatarHider
there's other mods that are harder to find such as