I take it from the comments here no one owns or owned an i5 8400.As much as I agree with the comments here forget about fps its important to have a well balanced system.
The op has a 3070.The i5 8400 is two gens old and its also locked @3.8ghz with only 6 threads. That is bound to bottleneck anything more powerful than an rtx2060 in a cpu bound game let alone enough to bottleneck a 3070.
imo its always a good idea to have a faster cpu rather than having an overpowered gpu with a low tier cpu.