Unless things changed since I played through it (which matched what I read about it at the time), Mario 64 will MOSTLY run fullspeed without overclock. There are certain areas like the submarine which need overclock to be fullspeed. I wouldn't expect lag all the time without overclock, unless something significant changed with the emulation in the last couple of months.
You should notice it actually just happens in the "widescreen" area at the sides of the screen. The game isn't programmed to use that area properly so objects get cropped to the 4:3 area. IDK if there are any
cheat codes to adjust that, but if not you can either live with it or set the screen display to 4:3 and have black bars at the sides instead.