it will make pretty much everything faster...

This is just not true. Environment variables are only going to be used by programs that are looking for them specifically. Putting them in your .bashrc as you have done is going to make them only available in contexts where that file is sourced, e.g. interactive command line environments.

...at the expense of breaking some commands here and there...

You probably experience this because you used a single > to overwrite your .bashrc entirely with that single command. Anything that was in that file before is now gone. Using >> will append rather than overwrite.

That variable in particular is probably one used by mesa, a 3D graphics library. It's only going to be used by programs that use the mesa library. I don't know what it does exactly, but there will be documentation somewhere.