Avidemux 4K editing: audio/video sync problem

[Hansenaz]

I'm a complete newbie here. I make simple Youtube videos with an old cell phone and Avidemux editing.

I decided to try making 4K snippets yesterday, and today, when I try to edit the snippets together I see a huge video time lag (video lagging audio). Apparently the video processing can't keep up. BTW viewing the snippets in Media Player there is no problem. Since I never had an issue when shooting 1920x1080 I conclude this is a problem with Avidemux and 4K video and my computer. (Right?)

Is there a nice solution? All I can think of is to maybe downsize all the snippets to HD before editing. I'd rather not (about 100 snippets). I've played a bit with video card setting (automatic, Optimus, Nvidia only) but it doesn't seem to matter.