You dont buy a card to test at low res tho do you, ur gpu isnt going to be needing to work hard for 1200p.

Heck my vega 56 probs do about the same fps at that res.
Btw is it true u just have 8gb of ram? That could be a issue. 16gb would be min for a gaming rig like urs defo.
Ask guest to run at ur native res using dsr then compare results.