Also, maybe try running the game with every optional program like Shadow Play switched off and killing all running processes which aren't neccessary. For example updaters and stuff like that. Just to make sure it's not some incompability. Remember the game is old and as someone already said it is horribly coded in some parts, which might cause compability issues with heavily optimized software of today.
You could also run the task manager while playing the game and monitor the ressource usage. You get graphs for each component (GPU, CPU, RAM) and could watch for peaks to find out exactly, which component causes the lags or is the bottleneck.
Remember, a peak in usage (causing lag) doesn't always mean the component is to weak, it might also mean the game itself is spamming it with unneccessary tasks which causes an overload.