Our team has been chatting about this for the last few minutes, and one thing sticks out like a sore thumb
still extremely slow in the BIOS, mouse is still laggy
If we understand this correctly, the experience was on both motherboards.
This is somewhere that lightning doesn't strike twice. That is, unless something is trying to make you pay for your sins 😈
All joking the side, when we see this BIOS/mouse stutter problem with our clients, we can easily say about 90% of the time, it's the 3.3 volt rail coming off the PSU.
It could be as simple as tarnished pins (1/2/12/13) on the ATX 24-pin connector, or what we find more often, the modular connector at the PSU. We find this rare, but we don't like the discount it as an issue.
But most of the time, under testing, we find that the 3.3 volt rail is wobulating (regulator break down causing DC voltage to alternate 2.6~3.4 volts, for example), or a diode has a weak spot and is letting AC voltage ride the 3.3 volt output.
Most of our clients that have a build like yours have the following to items in their extended FGT (Franklin Gamers Toolbox) tucked away under their desk, under their bed, or in their closet.
One is a simple ATX voltage tester and the other is a c-h-e-a-p Chinese power supply they keep for our testing and as a back up in case they need to RMA their current power supply.
They're both the cheapest anti-headache tools to have on hand, among other toolbox items.
The GPU, also has access to the 3.3 volt rail, and can easily be a problem too. We're not sure of your older configuration, but the new 600 Series chipsets are far more demanding on the 3.3 volt side, mainly because of chipset management.
So eliminate the GPU first (we highly doubt it's the issue, but once again, let's take Murphy out of the equation), and if you can bag, borrow, or steal a PSU to connect externally for a quick trial, that would be the next process of elimination.
You may also want to burn some candles, strip naked, dance around while sacrificing a chicken, just in case some entity is causing this 😊 your old setup may be holding a grudge...
MaxProAndU Team
You've proceeded properly so far.
Here are a few things, based on your post, that we found in dealing with our clients.
First, verify voltage of your replacement CMOS battery. We seriously do not believe this is the issue. We've just learned not to ASS-U-ME in these cases.
Second, disconnect the power button wires from the motherboard, so we can rule out an intermittent power button. You would be a surprised how many headaches it gives people. Simply start it with a "pin jump", and see if the problem is gone.
The rest of your prognosis makes our team question the earlier PSU replacements. Without knowing the prior PSU history, it is highly possible that the 3.3 volt rail in the replacement PSU is faulty.
If this is the identical issue that recreated the PSU replacement originally, something's compromising the motherboard or the motherboard itself is defective in some form. The number one source of compromise is the GPU, followed by a monitor. Both have access to the 3.3 volt rail. Although rare, a poorly constructed DisplayPort or HDMI cable can be to blame.
The best test here initially, is to use a new DisplayPort cable, and a different monitor. Even if you have to connect it to a television.
You can also try a tool like this one our clients keep on hand
As it does reduce the number of headaches when it comes to PSUs, but isn't the best on the 3.3 volt rail due to the inability to create a load. Still, it's handy to have
Hope this information is helpful
MaxProAndU Team