A recent VHLT specific tutorial with lots of pictures:
https://forums.svencoop.com/showthread.php/45301-Tutorial-Total-Map-Optimisation
(A few things are sven specific like the upcoming "wireframe color" command)
Few odd tips:
-If you have a detailed bush-model set it to func_illusionary with a clip brush for collision
-some details are best done with masked transparency images or for an always facing sprite use an Parallel Upright for signs or some trees
Legacy stuff:
http://www.slackiller.com/tommy14/rspeeds.htm
http://www.countermap2.com/Tutorials/tutorial0b30.html?id=2
https://twhl.info/wiki/page/Tutorial%3A_R_Speeds
https://web.archive.org/web/20000609203718/http://www.halflife.gamedesign.net:80/resources/fpschart.shtml