[Symmetry]

For increasing frequency the problem is we used to be able to just bump the clock speeds when the circuits shrank, but we've gotten small enough that the transistors start to leak now, so we have to drive them with less voltage to prevent the chip from overheating, meaning the clock has to come back down </huge-oversimplification>.

There are some possible ways forward. If we're sticking with transistors we might be able to switch to a material with better electrical properties, but that needs lots of research before it'll be higher performance than silicon.

There are also funky non-transistor based ideas for doing computation, like using DNA or nano-scale clockwork or ballistic electrons. I have no idea how feasible that stuff is.

In the shorter term, computer engineers are finding ways to turn those extra transistors into better single threaded performance by better prefetching, branch prediction, and re-ordering your instructions so that more than one are executed at a time even though you never thought about parallelism when writing it. That's why a modern computer core is much faster than an old Pentium 4, even though the clock speeds might be the same.

The problem with that is that using more transistors tends to provider at most a O(sqrt(n)) speedup, whereas adding more cores potentially provides an O(n) speedup.