GHz represents the number of cycles per second, not the number of bits actually processed per second. On different CPUs the same instruction can take a different number of cycles, a different number of the instruction can be in flight at the same time, a different number of several different instructions can be issued at once, a different amount of data can be pulled from cache to feed these instructions, a different quality of instruction reordering and branch prediction, and so on.

As an example a single thread on an x64 core of an old Pentium 4 661 @ 3.6 GHz benchmarks at 315 with PassMark while a single x64 core of a current 285k @ 5.7 GHz turbo benchmarks at 5195. Some of that also comes down to things like newer RAM to feed the CPU but the vast majority comes down to the CPU calculating more bits per clock cycle.