I just updated my M1 MBA, it feels basically the same as 26 performance wise (that being said, I didn't notice a performance difference between 18 and 26 either).

I think the appearance is actually worse. They haven't addressed the giant padding areas 26 added to everything, but they added a faux-3D effect to the Liquid Glass. Even if you set the glass to opaque, the faux-3D remains. I don't mind faux-3D (I thought Windows 7 and OS X 10.6 looked fine) but it looks out of place here because the majority of the UI still uses flat design. It looks like if you installed a Windows 7 theme on top of Windows 10, then cancelled the install midway through so it's half applied.

The light theme still has the contrast blown out and hurts my eyes to look at, so I still have to use dark mode even though I preferred light mode on 18 and below.