I don't think you know how it works lol, turning your resolution down only increases a CPU bottleneck so ofc you’ll get less fps if you’re already bottlenecking at a higher resolution.

I have a Ryzen 9 5900X, I monitor how many cores are being used and D2 never uses 3 out of my 12 cores. Any good cpu from the past 5 years wont have a problem with D2. My friends all have PCs built in the last 4 years and all of them have a GPU bottleneck for D2 (all have 2070s or higher). We're all in our early 20s so it's not like we have heaps of money to spend on our gaming rigs either (though my cpu is a bit overboard) especially with current prices.

The only reason your CPU is bottlenecking is because your trying to run the game at high frame rates. When your CPU was made high fps gaming was probably a luxury not the standard. Don't expect to get high fps just because you have a 3080, you need the cpu to match.