I found a simple way to enhance the framerate on 3.5 tft waveshare screens at least, at the cost of time for research and tests!
I ended up with a line to change in config.txt:
> sudo nano /boot/config.txt
==> Find the line containing "dtoverlay=waveshare35a" and complete it as follow :
I don't like to have the power cable downside cause I can't put the pi on its edge, so I flipped it with the rotate and swapxy options.
After numerous tests, I found that the best compromise between framerate and color depth was to set speed at 41000000 and fps at 50. For some reasons, if I change the speed to 42000000 or more, the color set fallback to 4K colors or so. Idem with fps.
>>> No more laggy display!
On an other hand, I unfortunately keep being stuck with the backlight issue: impossible to dim it nor switch it off. I guess they omit to set an option or a gpio pin to deal with. A huge mistake according to me, who doesn't care about that? It makes the pi warm up and it consumes useless extra power when left on.