Leak of Handles?

Opus was sluggish and searching for a cause I saw it had 12K open handles and closing tabs only increased it. After restarting the process, the number went down to less than 2K.

Could this be evidence of a leak?

It may be, but isn't conclusive. Various things may create handles to cache things, so the count won't always go back down. If it keeps going up indefinitely when you do certain things then that might indicate a leak.

If there is a leak, it's very possible it isn't in Opus and is instead in a shell extension. They're usually what ends up being the cause of that kind of leak. (But it is also possible for Opus to leak handles, of course. It's just less likely, from experience.)