What’s the best upgrade option for my i5 6600k?

Your upgrade path really needs to shift to something new. I’d suggest considering the 12100 or 5600; those will both give you a solid performance boost without too much hassle.

It's worth noting that you'll need a new system anyway, as your Skylake CPU isn't supported anymore in Windows 11. Plus, Windows 10 will reach the end of support in October this year, so that's another thing to keep in mind.

I'd say a 7700K isn't really a big upgrade and might be overpriced even as a used option. If you want a true upgrade, consider getting a newer CPU along with a motherboard, like a Ryzen 5600 on an affordable AM4 board. That should give you a significant boost without breaking the bank!

Honestly, many people say upgrading to an 8-year-old CPU isn't smart. If you're considering a budget build, even the $600 AMD systems on PCPartPicker will feel much faster and give you a better experience overall.

If you're set on saving some cash, go for a 5600X on a B550. It's the best bang for your buck, plus you can reuse your DDR4 RAM. However, if you want future-proofing, consider the 7600X on a B650 or B850 board; it might be pricier but it's worth it if you want to upgrade down the line.

Honestly, the i5 6600k is pretty outdated. You’d be better off looking at a new platform with 6-8 cores, depending on your budget. Once that's sorted, you can upgrade the GPU later if needed, but a modern CPU should alleviate the lag you're experiencing.