Question – New Win10 build, taskbar is “mushy”

Just built a new decent spec machine. I have built my own, many machines since 1998. But this machine is odd. Resizing the taskbar in Win10 feels like dragging marshmallow. I use the taskbar vertical up the right hand side of my monitor. Dragging it wider takes a second for it to move, then it sliiiiiiides out, but then continues past where I moved my mouse to. And if I drag it back there’s noticeable delay. I would attributed this to slow hardware but my specs are:
9950X3D
9070XT
128GB 6000mhz cl36 DDR5
4TB 990Pro NVME
Gigabyte Aorus Elite B850
EXPO is enabled on the RAM

I installed Win10 Pro, cos Win11 is absolute gankware. It is NOT an option. I understand W10 is EOL, but it should still work, right? I’m willing to take my chances. The ISO came from MS, using their builder, onto a brand new 32GB thumb drive. I have disabled the iGPU, though it was still there as an error in the device manager, until I manually pointed it at a driver and it went away.

Windows installed fine. I installed the AMD driver thingy which installed the chipset driver and the latest GPU driver. The PC is working to spec. Without OCing I get 43,606 in Cinebench r23, and 7315 in 3DMark Steel Nomad. Both bang on the money for my spec, apparently.
But the taskbar, with transparency enabled, feels mushy, like there’s a driver issue.

I’ve reinstalled the drivers direct from AMD and Gigabyte.
I’ve tried a different monitor, and the other HDMI port on the card, to be safe.
I tweaked the power options in windows.
I tried to force explorer.exe to use the GPU in high power mode.
I emptied the shader cache.
I changed shader cache from AND Optimised to FULL.
I downloaded a program to turn MLO off.
I scanned windows for corrupted files, it found a couple and fixed them.

Turning off transparency and it’s instantaneous and pixel perfect. But with it on, it’s hard to live with and a worrying indication something else is wrong.

App windows don’t seem affected. Dragging them around is still fast, as is resizing them, without tearing etc.

Does anyone have any further ideas?