Recently my relative went and bought an RTX 4070 even though his setup includes an Intel 6770 and a motherboard with 16x PCIe 3.0 lanes, which IIRC can't accept more than one generation newer CPU.
Turns out the performance penalty is in the low single digit percentages - if at all.
Some 10 years ago, I had a i7-2600k at home and my motherboard died. Of course this happened Saturday after the shops were closed, so had to use some old left-over parts.
Didn't have a compatible MB but found an old MB with a dual-core Celeron on it, and 4GB of RAM. So installed that, paired it with my GPU, an AMD HD 4870, and a spare SSD with a fresh copy of Windows 8.1.
Even with the limitations of dual-core Celeron and only 4GB of RAM I could play most of my games with quite acceptable framerate. General usage was surprisingly fine thanks to the SSD.
Was quite shocked as well to find how usable the whole setup was.
Turns out the performance penalty is in the low single digit percentages - if at all.