I’ve been using KDE Connect since march with Fedora 39 and now with Fedora 40 (both KDE spin) and I do not have a good experience with remote input, I’ve tried several phones (Note 20 ultra, S22 ultra and iphone 13) and it doesn’t work well on any of them. When I slide my finger on my phone the cursor moves with a lot of latency and the movement is very choppy.
I’ve already tried opening ports 1714-1764 (TCP and UDP) both in router and firewall, and completely disabling the firewall and I cannot get it to a usable state.
I’d like some advice to see if I can make it work, because this is a feature I really like.
Currently I’m running Fedora 40, kernel 6.9.7 and KDE Plasma 6.1.1 (wayland session).
Please let me know if I should include any more info about my system, any help or advice is welcome.
I have been a big user of KDE connect for a long time, and it usually works flawlessly, I do not know if it has had an update and I did not realize, but recently its become inconsistent with remote input for myself also.
Sometimes it will lag like crazy but then randomly work flawlessly again for a moment, but only until I stop, the moment I try move the mouse again it will be choppy.
It is quite frustrating not knowing how to resolve this, I have tried the port forwarding also, and resetting the cache for the app, and resetting the apps config on the pc end also.
The mouse is super choppy and almost unusable 
I use this app a LOT because I use it on my steam deck when its in a dock I have as I do not currently have a mouse and navigating a desktop with no mouse in modern times is difficult without a tiler.
Here is my OS info
Plasma 5.27.5
SteamOS Holo (Arch)
Kernal 6.1.52-valve1
If anyone else could give some input to this it would be greatly appreciated.
This appears to be the case for the input of the keyboard too which previously as I typed the characters would instantly appear, now they have a slight delay too
Edit: I just updated my SteamOS to beta, which in turn updated KDE connect from kdeconnect: extra-3.5 (23.04.2-1) to local (23.08.5-1). Not sure if that small update fixed things or the whole system update, but for me it no longer is laggy, perhaps OP can try to update KDEconnect?
Unfortunately a simple update did not do the trick for me, I’ve had this issue for several versions (since Fedora 39 with KDE 5.27 and kernel 6.7) and non of the updates did anything, not even a clean Fedora 40 install with KDE 6. So I guess I’ll have to wait to be able to use this feature.
So I ended up getting the same issue once more, now this time around I think I may have found a potential culprit.
Have you at any point modified your autologin file? or switched to using the x11-plasma session persistent?
I usually like to keep my steamdeck booting into the gaming mode and in doing that I make the steam deck stick with the plasma session and not gamescope/wayland.
After an update recently (when I made my recent post about it working fine now for me) it was working fine (my theory is that it reverted my perma plasma session, upon once more setting my steam deck to only boot into desktop mode it seemed to bring the laggy input back.
So I thought hmm, the only thing I have changed is this, so I set it back to normal, and boom it no longer lags.
Perhaps KDEconnect is getting confused between the x11 session and wayland, and that it doesnt work well with wayland?
TLDR: Try (if you have your session set to use plasma persistent) switching to gamescope so that the deck forces the change in session, then going back to desktop see if that helps?
If you dont know how to do this type this in your konsole
steamos-session-select gamescope
Another thing to note that I have noticed.
If steam is not running KDE connect will behave badly, sometimes it tries to update and cant due to a network issue and does not launch, if you launch it once you have internet, it should work better.
Give all those things a try perhaps.