Microsoft's antivirus is one of the worst ones these days, in terms of negatively affecting software/performance. Although it may not be the cause in this case, of course.
If you want to diagnose it, using Process Monitor to see what's being queried and if any of the actions is taking unusually long is the best next step.
Yes, that is strange and makes me suspect something weird is going on with the machine. E.g. I've seen weird pauses like that with a new motherboard with immature BIOS/drivers that didn't handle switching power states correctly and kept going into a low-power mode for too long, making the desktop randomly pause for a second. (In that case it affected Chrome rather than Opus, but the same kind of thing could affect anything, and whether it was triggered or not depended on a lot of small factors like how much the mouse was moving or what it was moving over just before doing something else.)