We were stuck with 33MHz PCBs for a long time as people kept trying and failing to get 50MHz PCBs to work. Then Intel came out with the 486DX2 which allowed you to run a 50MHz processor with an external 25MHz bus (so a 25MHz PCB) and we started moving forward again, though we did eventually get PCBs to go much faster as well.
The Transputers (mentioned in other comments) had already decoupled the core speed from the bus speed and Chuck Moore got a patent for doing this in his second Forth processor[1], which patent trolls later used to extract money from Intel and others (a little of which went to Chuck and allowed him to design a few more generations of Forth processors).
> We were stuck with 33MHz PCBs for a long time as people kept trying and failing to get 50MHz PCBs to work.
What is the current best symbol rates we get on PCB traces? I know we’ve been multiplexing a lot of channels using the same tricks we used with modems to get above 9600bps on POTS.