fastest speeds depend on two things. First the manufacturing technology used (smaller is faster because parasitic capacitance is smaller, although finfet is getting to have large parasitic resistance). The 2nd factor is the datapath between flip flops. Some architectures handle very fast clock rates because the data path is simple (or pipelined). Others are quite slow. I've seen some architectures top out at 500MHz when another architecture is running over 1 GHZ in the same chip.