I just had this right now changing from F-16 to MiG-21.
I checked the MSI Afterburner, but both CPU and GPU were pretty low in load - however, in Task Manager I noticed that Steam's VR Compositor took the bulk of the 60% GPU load. Alt+tab didn't fix it. Also, this only happened since yesterday I went through my PC/DCS settings to improve performance using tips at VR4DCS.
(BTW in MiG-21 training missions the RSBN needle didn't work, but that's another problem...)