After starting BTT, iCloud starts churning through a lot of data (uploads/downloads) and fairly quickly bird and fileproviderd processes climb to high CPU usage. Quitting BTT, all returns to normal after some time (depends on how long BTT was running).
It seems BTT hits files in my iCloud such that iCloud thinks these are updated/changed somehow and the upload/download cycle begins.
This is new behavior that started (at some point) after the latest macOS 15.4.1 security patch. Would be nice to hear of a solution as I cannot run BTT as it is now.
If you had it enabled, it is almost definitely that. Unfortunately it is super experimental, you might need to do a full clear of the BTT installation if just disabling doesn't help.
You can also try disabling it in System Settings => iCloud => See All => BetterTouchTool