@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?

the way you're doing it already - z_pi_crt.

@BuZz thank you...i have fix It with shader update

@megaJekan as said in your other thread, 480p is unnecessary for a pi3b+ and pcsx. psx emulation has been running full speed on pis since the pi2. if you're getting slowdown at 1080p and NO shaders, you have a system issue.

as for curvature shaders - zfast should be full speed at 1080p on most games and is the fastest one available. they won't look at 480p as it's just not enough resolution for that effect to look good. it's not enough resolution for a non-curved scanline shader...