Windows defender has the uncanny ability of spiking up its resource usage at just the wrong time. Add in MS teams into the mix and the system just starts screaming.
I know nothing about how defender works internally but it seems to do a synchronous scan on every single file access and an extra long one on a DLL or exe. It seems to cache them but throws the entire cache away on restart. The behaviour is worse on C than another partition even if the partition is on the same drive. Anything that accessed a hunch of files on startup will bring your computer to its knees on first launch.