AFAIK it depends on output resolution, rather than display tech per-se.
So even on a low-res 15Khz, using frame_delay on super resolutions should tax the GPU more than with low dotclock modes (logically?)
Though maybe with a strong CPU (which is your case iirc) you'll never see any difference in frame_delay performance, even if the GPU is low-end.
Also stating the ovious but MAME drivers, even some of the 80's stuff, get heavier over time as for instance sound emulation improves, so if the CPU load becomes significantly higher than usual there's maybe less time allocated for frame_delay and the GPU to deal with it.
Older toaster-tier Groovy PC setups that haven't been updated in ages, might struggle if updated today, even if we're still in the 15KHz CRT realm.
Yeah I agree R5, R7 series aren't bad at all and not-too-old.