consider upgrading from i5-6400 to i5-9400f for better performance
consider upgrading from i5-6400 to i5-9400f for better performance
It seems like you're considering an alternative that's costly and only offers temporary relief.
It all hinges on the specific game. If the 2600 struggles with the 1070, then the 9400f is out. No one has complained about the 1600 either. Checking benchmarks shows no red flags that suggest single-core problems. There haven’t been any recent forum complaints over the past year. The 2600 also sidesteps the thread issues the 9400f faces. It seems like a more reliable choice now and for the future.
Yes, that's not the way it functions. The 6th generation might be compatible with the same socket, but it doesn't support the 8th and 9th generations CPUs. The 6th and 7th generations share the same motherboard. However, the 8th and 9th generations need 300 series 1151 boards. You'll need an H310, H370, B365, Z370 or Z390 board. Anything from the 7th generation or earlier won't work.
Based on this reasoning, any CPU seems like a temporary fix. I understand these the ones I mentioned are often overvalued. If a good deal came under $140, it would definitely make sense. You never really know what you might find in certain second-hand shops.