Hello, I've updated to 17.1 early this month and so far I've got one issue only: the lag when choosing and uploading a local file, especially images from the pictures folder on Files. Used to be pretty snappy on 16.3.
Am I the only one experiencing this?
Where do you get this error? In a browser or in any other specific program?
If it's a browser (and default Firefox in Z17) then I suspect it could be due to it's flatpak package. In Z16 Firefox was in native deb/apt format instead of flatpak. You can install Firefox's deb format using this guide -> Install Firefox on Linux | Firefox Help
Before that, first remove the current firefox using the following command in a terminal window:
flatpak uninstall org.mozilla.firefox
Same I can guess for other specific program if you are using any. If this is so then try installing the deb/apt version of that program and check if the lag still happening or solved.
uh, do you have any reason to suspect this considering Firefox native also uses portals? 
I cannot speak for @littlekun, but I would suspect the same if the browser is the culprit due to xdg-portal being an additional authentication step. This can create lag rather than failure.
That's just a guess and the first thing that came into my mind as the OP had mentioned the process was snappy on 16.3. I don't use flatpaks so don't have much idea how they handle the interactions with the other apps but I do know that Flatpaks need additional permissions manually to get the best out of it and this was the reason for my suspicion.
I know there could be other reasons too. Once the OP confirms and shares where they are getting this issue, we may get better clue why it's happening.
No, we got Portals for tihs, and many applications use them now out-of-the-box, with or without Flatpak, to get files without exposing access to the whole filesystem. It's entirely automatic in that regard.
xdg-portals aren't particularly heavy to run, and they run int the background anyways, so unless they have a really low-end device I wouldn't expect any issues to stem from this.
Hello, I've made a quick screen recording: Bubbles - Teamwork. On your time.
I'm using Vivaldi browser on a pretty solid machine (AMD Ryzen 9 3900, 32 GB Ram, RTX 2060).
Actually I have this same thing too.I just put some photos up online and I am on 17.1 lite and it took much longer than when I was 16.3 lite.
I have no idea how to fix it though sorry.