I'd been picking at this for a few days but it's not a Proton issue I have just found out. Gnome opens an invisible emoji picker with ctrl + shift + e, which isn't mentioned in any hotkey UI.
Enter below into bash to remove, in case anyone lands here in future.
gsettings set org.freedesktop.ibus.panel.emoji hotkey []
proton experimentalx1 2023-02
Compatibility Report
System Information
I Confirm:
[x] I haven't found an existing compatibility report for this game.
[x] I have checked for system updates.
Symptoms
When sliding and interacting with an object (Ctrl + W + E), be it a door or picking up an item, there's a second or two of frame drop and stutter then keyboard inputs stop registering. Pressing escape returns normal keyboard functions.
I have also tested this by using the same combination of inputs but without an item present to interact with, and whilst there's still some frame drops the keyboard inputs don't stop registering.