I use UltraCompare only on Windows. So I cannot really answer your question.
Has UCX also the feature to poll for changes in compared files in background as UltraCompare for Windows?
There is in UltraCompare for Windows in Configuration (perhaps Preferences in UCX) at File Handling the setting File change detection which is by default checked. Well, this setting has nearly no effect on CPU usage according to Process Explorer.
UltraCompare v188.8.131.52 uses on my Windows machine between 0.50 and 075% of CPU on no comparison session loaded, between 3.50 and 3.85% on a text comparison session loaded and comparison already finished and UC window is in background, and 1.00 to 1.50% on text comparison session closed and UC still running in background.
When I watch with Process Monitor on Windows the file system, registry, network, process and thread activities and the profiling events for a minute on having text comparison session loaded and file change detection enabled, I see only some profiling events and every 20 seconds a thread exit which seems to be the file change detection thread. The threats count of UC on Windows is 18 (two more in the first two minutes after startup which could be for update check and cloud service check).
The process activity values of UltraCompare for Windows are quite okay in my opinion for an inactive Windows GUI application.
What do other GUI applications need on your Linux machine on running in background not taking browsers into account which always need a lot of CPU power even on running in background?
Best regards from an UC/UE/UES for Windows user from Austria