I know a couple of people reported a while back (maybe 6 months ago?) about MC lagging after it had been on for a while and had been switched to a different output (?). I have that same problem, which I noted here
https://yabb.jriver.com/interact/index.php/topic,125081.msg866294.html#msg866294along with an AV issue. I don't think they were related, as the lagging has persisted even after totally getting rid of Malwarebytes.
I have two monitors connected to my HTPC in two different ways:
the lagging one = HDMI to an HDMI Doctor to a Marantz SR5010 AVR and thence via a 2nd HDMI cable to an LG 1080p TV (not 4K)
the non-lagging one = HDMI to a standard computer monitor
My suspicion is that there's some conflict between an AVR setting and an MC setting because there's no lag on any other software when I'm connected to the AVR, only MC, which gets progressively slower till I have to restart it. I've played around with the AVR, turning this and that setting off, but nothing seems to help. I was just wondering if anyone had any suggestions about what to look for, stuff to try, etc.