Thanks for providing the log file. I don't notice anything different between the 2 runs, so it's not something related to a configuration.
Try removing pulseaudio completely then reboot the system, this might get rid of any PA interference.
@sad_muso you didn't say what emulator you're using. afaik even simple shaders are often very system bus intensive, so they're constantly fighting for bandwidth with the GPU, CPU, etc. afair, lr-genesis-plus-gx really hammers the system bus, so try using lr-picodrive instead, if you're using the former.
Are they CPU/RAM intensive or both? Does the Pi 4 cope with shaders better than the 3 B?
first part answered above. pi 4 will cope better.
Do overlays cause any hit on performance, specifically when used in conjunction with shaders?
yes, for whatever reason they seem to.
What is the least expensive way to implement curve and scanlines that is currently in existence?
Updated the RetroPie-Setup script and all OS packages and restarted the Pi. This got the Bluetooth to connect (unreliably, it still fails to register about 1/4 of the button pushes I make). Frame rates are still very poor but I think that might just be the state of RetroPie on Pi 4 at the moment.
No ideas from anyone? Disappointing... But I guess we can't expect to figure out a fix for everything. (especially if my friend is right)
EDIT: YEAH, my friend was right. Screen was dying. No longer functioning at all. Sorry to have wasted the time of anyone who read this - fix a dying screen with configuration changes is....kind of impossible.
@Kentinator there are several threads concerning N64 Emulation with the Pi, but to say it with a few words: your experience nails it.
Mario64, MarioKart64 and a few others work well, a lot of other games do not.