First administrative: I tried the different recommendations in the pinned instructions for posting but none of those solutions worked. However, clearing the cache solved my UI lag problem. I hope that the pinned instructions post can be updated to include clearing the cache, probably earlier in the list too -- it is less disruptive than disabling extensions or clearing settings. I cleared my cache by renaming the "AppData\Local\Opera Software\Opera Stable" folder (not to be confused with the one in AppData\Roaming)
Recently I have had intermittent issues with the UI being extremely slow / laggy, to the point of being unusable. For example, if I select a different tab, it might take 1-5 seconds before that tab gets highlighted and has its content rendered. Interestingly, web page rendering does not seem to be slow. But any mouse or keyboard interaction is extremely slow. If I go to the address bar and type in an address or search, it takes seconds for my typed characters to appear. If I click the “O” menu on the top left, it might take 1-5 seconds to appear. Also, hovering with the mouse also has a long lag before the hints or menu selections or other responses occur. Interacting with web pages is slow, but only from the mouse to the page; rendering seems normal. None of my other apps or browsers are slow.
I tried deleting my cache (renamed “Opera Stable” to “bkup Opera Stable” within AppData\Local\Opera Software) - and the problem disappeared. UI is snappy again.
Looking in the cache_data folder I see there is one file called data_3 which is 90MB. I suspect this may be a problem but who knows.
Prior to deleting the cache, I tried the other recommendations in the pinned instructions post - renaming the profile directory, restarting my computer, closing all tabs, disabling extensions, etc etc. None of these had any impact whatsoever, but clearing the cache worked great.