There is a utility program to do that, and it will only make a difference on a G3 with an older video card. It will also make OS X look ugly. I cannot remember the name of it, and it will remove drop shadows. I ran it on my cube and noticed no apparent speed increase.
Check at versiontracker.com
It's called ShadowKiller (I think all one word) but you should realise that all menus and windows have an Alpha channel, and this program simply tells it to ignore that. The results can be pretty ugly, and it will only make a difference on systems that don't support alpha-masking on the video chip.
I'd consider another browser, IE is generally quick and stable but there are other options.
There is a utility called "Visage" that allows you to customize some things, including transparency. Check it out at http://www.versiontracker.com.
If it is THAT slow you should stick to OS9, until most software really works native under OSX.