The GHz isn't the main issue; it is latency. We were stuck at 3-3.5 GHz for a long while with dual core being super expensive.

Now we're at 6-6.2 GHz in turbo mode. Can we go faster?Likely, but I don't see 12 GHz within forseable future. Instead, internal parallelism within a core have increased significantly. The bottleneck is cache reload and flushing from/to RAM. Highly optimized programs /may/ fit in cache (assuming they don't get swapped out due to task switching) but most don't since they use large frameworks and/or are interpreted or runs on Windows that since dawn of time always take 2-3% CPU.

But there's a lot of programs that are unable to utilize multiple cores, so even if there's 20 cores available, 19 are unused and the last one is limited.

Lower voltage allows for somewhat faster clock speeds, allowing faster execution. Microcode based instruction sets (x64) benefit the most from internal parallelism due to the ability to begin an instruction before the previous has completed, while ARM CPUs generally split the entire instructions into independent parts that all are executed simultaneously, but there's a limit on how much that can be parallelized given a certain RAM bus width. Increasing RAM bus width, expecially external RAM bus width is likely where the biggest gain is.