This issue is bug DNA-68444 that was filed in March of 2018. Opera bought some 240Hz monitors just to test. Long story short, no one at Opera was able to reproduce the problem. Opera's UI was not laggy and it behaved the same as Chrome's UI.
Also, see https://forums.opera.com/topic/24026/ui-is-laggy-only-runs-at-60-fps-on-144-hz-monitor and https://forums.opera.com/topic/19574/ui-elements-are-laggy-and-animate-at-only-60-fps-on-144-hz-monitor.
There's also https://www.reddit.com/r/operabrowser/comments/59qbe0/no_longer_hitting_144fps/, which has a test that Opera passed when testing. But, that was in a web page and not the UI.
In short, this is unlikely to get fixed as no one can reproduce the problem.