High speed is not determined by clock frequency alone, the rise time of signals determines the spectral content of the square wave, this is determined by the knee frequency (Fknee=0.5/Tr, where Tr is 10-90% rise time), so with todays fast rise time signals you have to quite often employ high speed design practices for even quite mundane circuits.
Mixed signal circuits usually refers to analogue and digital circuitry on the same board.
High speed is not determined by clock frequency alone, the rise time of signals determines the spectral content of the square wave, this is determined by the knee frequency (Fknee=0.5/Tr, where Tr is 10-90% rise time), so with todays fast rise time signals you have to quite often employ high speed design practices for even quite mundane circuits.
Mixed signal circuits usually refers to analogue and digital circuitry on the same board.