Try these performance settings including the settings in the msfs20 settings guide (Link at the bottom of the page):
Performance Tips - FlyByWire Simulations Documentation
It worked for me. Before, I had very worse fps and stutters too and now I have even at complex airports up to 20-25fps (usually between 30 and 40 fps at airports).
Specs:
AMD Ryzen 7 7800X3D 8-Core Processor 4.20 GHz
64.0 GB RAM
Geforce RTX 4080