Recently I upgraded to macOS Big Sur and I tried to start up my Windows 3.1 VM which used MS-DOS, problem is MS-DOS would load but it would always get stuck at "HIMEM is testing extended memory" I even tried to create a fresh DOS VM and the problem persisted, I've left everything at default settings, 16MB of RAM, 2GB hard drive.
So after leaving it for a while, the DOS prompt does indeed come up but it took WAY more than 65 seconds to actually reach the proper prompt, and it does this every time it reboots or starts up, it's quite annoying to say the least.
I'm having the same exact issue since upgrading to Big Sur. I'm running VMWare Fusion 12.1.0. My VM runs DOS 6.22.
It takes about 5 minutes for the HIMEM memory test, but it eventually does finish.
This does not appear to work for everyone, but down here I've set the virtual hardware to version 6 (Settings -> Compatibility) instead of version 18 and now my Himem testing is only a few seconds instead of a long excruciating time.
(Host OS macOS Big Sur 11.3 b3, VMware Fusion 12.1.0)