Just putting this out there in case others are having the same issue. So treat this as an observation.
So recently I rejoined two instances of Syncthing, essentially putting it all back to how it was on 1.3.4. Since 1.5.0, im finding that occasionally the computer will be so locked up that I have to pull the plug. I have had to hard restart twice in the past week. The GUI has been unresponsive several times.
I have also noticed my database folder is now hovering between 5 - 10Gb and CPU threads earlier today was 556. Which has been mentioned and the response was due to me using USB drives. Which I still use them, I have also invested in an i9 / 32Gb / 14Tb (large sync) data drive but gut feeling is 1.5.0 may not be able to scale to large volumes of data / lots of folders in a windows environment.
I’ve set concurrency to 10 (was -1) and will see how it goes, but I have the horsepower to keep on top of things, but St is somehow locking things up.
As I said, observation, not a bug or support request.