Increase the counter factor, use low resolutions, and disable fancy effects such as texture filtering and anti-aliasing. Have a look at the suggestion for slow computers on
this thread.
You should try out some different video plug-ins, too. I recommend that you just download most of the popular ones (Jabo's, Rice's, Glide64 + Wrapper, glN64, Direct64) and work out which ones provide a good speed-quality balance. You may find that 1964 is a little faster, aswell.
I wouldn't get your hopes up about Pokemon Snap. As far as I know, the only way to make it work properly is with PJ64 1.6 + Jabo's Direct3D 8 plug-in with framebuffer stuff enabled. With this configuration, you'll only get a few frames per second.