I assume you're talking about the preview windows, and not when playing a video you produced in PD13.
4k and 120fps take a lot of horsepower to edit in full definition in real time, so the best choice - even on a powerful machine like yours - is to reduce the preview resolution to High AND use shadow files. Wait until the shadow files are finished processing (the yellow ready in each clip in the media library turns green) before you start editing.
YouTube/optodata
DS365 | Win11 Pro | Ryzen 9 3950X | RTX 4070 Ti | 32GB RAM | 10TB SSDs | 5K+4K HDR monitors
Canon Vixia GX10 (4K 60p) | HF G30 (HD 60p) | Yi Action+ 4K | 360Fly 4K 360°