Only upgrading the CPU won't help much, because your mainboard won't support much faster CPUs. And with a newer mainboard, you will probably also need new RAM. I guess you have DDR3 while current mainboards usually need DDR5. And you might need a new power supply if your power supply doesn't have the right connectors or doesn't supply enough power for a new CPU. A good value option might be a previous generation AMD processor like the Ryzen 5 5600, because its mainboards are cheaper and use DDR4 RAM which is cheaper as well. But of course it won't be as fast as a current generation AMD or Intel processor. You could also look for used parts/computers. I strongly recommend to look at single-core benchmarks from multiple sources for any CPU which you consider buying. Don't be fooled by brands like i7 or ryzen 7 or GHz values, because they are only meaningful to compare CPUs in the same generation. Newer generations outperform older generation CPUs from a better brand/with more GHz. For eu4 the single-core speed is the most important, because eu4 can't make good use of the other cores(but you should still buy a CPU which has at least 4 cores/4 threads). Somewhat of an exception are the X3D processors from AMD(5800X3D, 7800X3D, ...), because their bigger cache is said to speed up paradox games by quite a bit, so they should outperform other CPUs with similar single-core benchmark results(but I have not seen any eu4 benchmarks, so I don't know how reliable that is). But they are still quite expensive.
If you play other games besides eu4, you should also look into what they need when considering a new CPU.