Kdenlive is amazing though, especially the timegraphs for applying effects, and the clip proxying which makes it amazingly smooth for large projects on old laptops. I don't think any of the other free video editors come anywhere close.
Even on a pretty powerful system, having a project timeline longer than an hour made the entire interface run at ~5fps for me when adjusting cuts. After that, I started using DaVinci Resolve, which definitely still has problems, but runs like nothing else I've seen.
Did you use proxy clips? It's not on by default[0].
[0]: https://userbase.kde.org/Kdenlive/Manual/Projects_and_Files/...
My experiences were on Linux btw (Ubuntu 20.04) using well supported hardware.
He loves learning how to use effects and titles, creating lots of cuts, and intercutting between different tracks. He even did some lightweight animation using it.
It's the most similar to the old-school feel of Premiere and Final Cut, and has given him a great foundation in non-linear editing and 2 years later he absolutely flies in it!
It was easy enough to figure out how to use it and I've got about zero knowledge of video editing. To be honest, I should really change to using ffmpeg instead as OpenShot can be a bit of a resource hog when exporting the video.
I have found the cross-platform support to be useful as I mainly use linux, but the more powerful PC that I've got is running windows (for running games).
Edit: I just opened the project up because I was curious about the RAM and vRAM. When it's idle it took 1.5GB of RAM and 1GB of vRAM. Scrolling through the project very quickly pushed the RAM up to 4.5GB and vRAM up to 2GB.
I got some crashes and random slowdowns; on par with other open source editors.
I found the UI unintuitive. For example: I tried to put one video next to another one (a presenter, with slides captured from the projector). I had to crop the presenter video but also adapt it to the height of the screen. The thing is that when I tried changing the crop horizontally the height of the video changed, as if the editor was trying to preserve the rectangle's proportions at all costs.
The one I always come back to is Blender. It too has serious issues, but it is actually improving fairly rapidly. In the latest releases you can scrub through video without it crawling to a halt (wow I know right!) and it has a non-tedious way to add text, etc.
Basically its interface is ... well if you've used Blender you know what it's like. But it does at least not crash, and you are unlikely to run into "you can't do that" issues, even if doing some simple things like changing the speed of a clip involves several YouTube tutorials and a bunch of caveats.
So yeah, open source video editing still sucks.
The user interface could be easier, for example deleting "ahs and ums" and silent time is a bit tedious. Also there are many output options, and it's confusing for a light user which to choose.
I've also used Lightworks, which is much nicer, but not free (except for 720p). Many of the non-free options want you on a subscription basis.
It actually has quite a lot of functions, and effects, but the UI/UX is so so in my opinion.
Video rendering and export is fast enough which is a good thing, even when using effects.
The real test with most of these authoring tools is text support and how comprehensive it is, or not, as titling is an important aspect of video editing.
So how's the text support?