Any chance you're using Discord while player? We've had a few similar reports from players who saw improvement after disabling the Discord overlay specifically. Lmk if that helps at all.
Can confirm that DX12 is required as of the mid-season update. We dove into some of the reasoning as to why we had to move away from supporting both variants with our first comms back around the launch of From the Rift. Some more details are also in our latest patch notes. Keep us posted if this poor performance continues after the initial compiling and we may have some suggestions to help.
From JP Community
Is there a particular reason why skill-based matchmaking is not applied in Mixtape mode?
Can confirm that DX12 is required as of the mid-season update. We tried to give folks as much notice as possible about this change both in and out of game, with our first comms back around the launch of From the Rift. Some more details are also in our latest patch notes. Lmk if your PC supports DX12 and you're unable to get the game to work with the added callouts in the PN.
Have you tried disabling Adaptive Resolution FPS Target in the Video Settings (more specifically, setting it to 0)? If not, give that a whirl and see if it helps at all. This has been flagged internally either way.
From our ZH community:
Since the SBMM mechanism causes matches to become increasingly unbalanced with fewer players, what measures could be taken to bring more people back to play?
From our ZH community:
Why is the match intensity in normal matchmaking noticeably higher than in the ranked?