Well, I didn't mean to pollute this SDL2-related thread
I have no idea what's wrong, I made many tests and get no 100% perfectly satisfactory result, although it's not that bad...
I use the proprietary Nvidia drivers / Kubuntu LTS 64 bits / latest KDE / bad GT 320M GPU and bad Celeron CPU on a several years old low end ASUS laptop. I tried to downgrade drivers to the legacy version, no difference. I use triple buffering in xorg.conf for kwin : disabled, no difference. I disabled composition in kwin : no difference. I disabled Twinview to make sure the actual screen frequency is the one reported by xrandr : no difference. Actually, when using "waitvsync" and "syncrefresh" I get a 50% performance drop. When not using "syncrefresh" but using "waitvsync" and "refreshspeed" I get a supposedly 100% framerate but it doesn't feel 100% smooth and there's a huge input delay. The best result I can get is with no option enabled (no refreshspeed, no syncrefresh, no waitvsync, compositing enabled), which leads to some tearing but at least it's fast and smooth
Well, I don't really mind that much, it's not a machine I use that much for MAME anyway -- I get a perfect result on a NUC.
Thanks for listening and sorry again for annoying everyone