as far as i recall the shader preset includes bilinear on/off, so you should not touch it and let the preset decide

@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.

Better late then never I suppose

https://github.com/libretro/RetroArch/pull/14628

Many shaders include options to alter the effects they produce, including brightening the picture. Have you tried investigating that? Details for crt-pi on what you can change and the easiest way to make those changes.

@Marinho Great thread, thanks for the info. I've been noticing the same thing here. Playing with shaders causes the system to run really hot. I thought it was just cause I was playing PSX games. But when the thermometer started showing up with SNES games I knew something was up.