>My test suite currently runs in ~6 seconds on 9700K

Absolutely nothing. 6 seconds is about the time it will take you to tab to your terminal, press up arrow, find your test task and run it. There's no amount of money that makes it go from 6 to 3, and no world in which there's any value to it.

In addition, upgrading to a 13900K means you're playing the Intel Dance: sockets have (again) changed, in an (again) incompatible manner. So you're looking at, at the very least, a new CPU, a new motherboard, potentially a new cooler, and if you're going too forward with CPUs, new RAM since Intel's Z890 is not DDR4 compatible (and the Z390 was not DDR5 compatible). Or buying an entire new PC.

Since you're behind a socket wall, the reasonable option for an upgrade would rather be a sizeable one, and most likely abandoning Intel to its stupid decisions for a while and instead going for Zen 5 CPUs, which are going to be socket compatible for a good 7 years at least.