Now the 32bit vms are very slow to the point of applications timing out with errors.
Well kjb brings up a good point, you did a BIOS upgrade, but whenever you proceed you should ALWAYS take note of the CURRENT settings, because a BIOS could reset them.
Or maybe the BIOS screen shows what the setting SHOULD be, but it's not active, so changing a setting and changing it back may 'reset' certain BIOS functionality.
So what may have happened is the BIOS reverted everything back to default, or introduced a different setting and that could affect your VM's.
So since the problem occurred after the upgrade, I'd say the BIOS changed something, or it could simply be a buggy BIOS, rather than fix problems it added new ones.
My question to you is did you check the BIOS notes first to ensure that a BIOS update was critical in nature? Because if it was just a BIOS update for the sake of keeping up with updates, that may have hurt your machine. If however they were critical fixes / bug fixes then I would call the vendor and ask them if they heard of any issues with that BIOS.
That may give you a clue.
Hyperthreading is turned on and always has been.
This has been a problem before, and the official answer as we have figured out through testing is Hyper Threading should be turned ON at the BIOS level, but DISABLED for ESX.
That way VM's aren't using logical processors, which is what is indicating your slow performance. What I suspect is that previous BIOS ignored or treated the hyper thread different, and NOW it's fixed. But 'fixed' meaning it should have been working like this all along, and you should have had problems, but since you just updated, now you feel the impact.
My suggestion is to disable the hyper thread in ESX, and see if the VM performance improves again. My final answer is it will.