If it works after a reinstall then whatever is causing the slowdown must be something changed by reinstalling.
That would be either registry settings damaged by some other program/installer, which the Opus installer puts back, or something within your configuration folders (/dopusdata, /dopusglobaldata and /dopuslocaldata).
Comparing the config folders of a fresh install vs copies of them from when the problem was occurring seems like a good first step.
If you hadn't mentioned Opus 9 then I'd think it was likely to be either a huge file collection being built-up by something or thumbnail caching going wrong, since those are the only things that really accumulate over time. (Or the saved folder formats for special system folders, but the way those are stored is more efficient in Opus 10, not less.)
I have seen antivirus scanners cause pathological slowness once the thumbnail cache files become large, since the cache files are stored in a format that they recognise as an archive and they start re-scanning the entire cache every time a change is made to it. (In those cases, you can usually tell the antirvirus scanner to ignore the thumbnail cache directory.) But Opus 9 stored its thumbnails in exactly the same way, as far as I can remember.
Similarly, collections might be huge or point at files that trigger a slowdown when Opus inspects them to verify their details (e.g. due to a virus checker, or pointing to a network drive is not there (which can cause problems in lots of other ways as well))... But none of that would apply to Opus 10 any more than it did to Opus 9.
Anyway, comparing the good vs bad configuration data seems like the best thing to do. That's all I can think of, given how vague the problem seems to be and that it is apparently fixed by reinstalling.