I think it's difficult to difine in between high or low speed board. However, I think you should strictly follow pcb design guidelines to optimise your design.
The definition of a high speed or low speed design is based on the switching rate of your I/O transcievers. A USB IO would be a low speed design compared to a gigabit Ethernet IO.
However the most important thing to remember is that even a 10Mhz design can be a high speed design in regards to PCB layout. Its all about transistor switching rate. If they are switching at low ms or ps ranges its a high speed design.
I would consider high speed PCB design anything where set up hold time of driver and/or reciver are taken into account during layoout, where property of PCB trace width trace spacing, trace width gettng calculate and estemated.
Above example with USB it is not relay low speed design try to play with trace width and trace spacing and see how it will behave