SNES & Genesis performance with video_threaded=false

[Smeg]

Pi Model or other hardware: 3B+
Power Supply used: Officlal Raspberry Pi PSU
RetroPie Version Used: 4.6, official image dated 20th April 2020
Built From:Pre made image
USB Devices connected: Gioteck VX2 wireless 2.4ghz dongle
Controller used: Gioteck VX2 wireless
Emulators: lr-fceumm, lr-snes9x2010, lr-genesis-gx-plus

I have a not-overclocked 3B+ on which I can run lr-fceumm just fine with the following settings:

video_threaded = false
video_max_swapchain_images = 2
video_driver=dispmanx
video_frame_delay =7

I can run all those same settings fine on lr-snes9x2010 and lr-genesis-gx-plus except video_threaded=false. When I turn this on I get severe stuttering,. almost like the game runs fine for 5 seconds then stutters, and this repeats. This seems to be a fairly common setting - have I missed something?

[dankcushions]

@Smeg this is why we have it disabled by default. see https://github.com/RetroPie/RetroPie-Setup/wiki/Input-Lag#unsupported-tweaks. on a pi4 there seems to be the CPU headroom to disable it on the 16-bit cores at least, but i've not done extensive testing.