It’s not that they are slow, it’s just that QtQuick and such are full GL rendering contexts, just have to keep some things in mind for those, and if you do they can actually be a lot faster than standard blitting rendering. 
Python itself adds a lot of overhead though, this is all more easily and efficiently done in native code. Python’s QT mappings are mostly just for ease of whipping together Qt UI’s, definitely not for speed. ^.^;
C++ is technically more portable than Python, you just have to compile it for what you want to run on though, lol.
That’s impossible, nothing in userland can do anything of the sort to the kernel unless there’s a major kernel or buggy driver bug. QT calls the native systems File dialog box if available, else it renders its own, all very trivial operations though; my guess is that when it called to open your system default file dialog box then ‘that’ is what was having issues. Do you know what DE you had at the time?
No clue but I’d bet something Greg did? ^.^;
EDIT: Yep, it’s something greg did, lol.