Bought a pre-built. Premiere playback is choppy, lagging, and unusable. Any idea what the issue is?

Build:
Operating System: Windows 11 Pro
CPU Type: Ryzen 9 7000 Series
CPU Speed: 4.5GHz
GPU/VGA Type: NVIDIA GeForce RTX 4090
Memory Capacity: 64GB DDR5
SSD: 2 TB PCIe
HDD: 6TB

AE so far seems okay, but Premiere can't seem to play a video or multi-tracks of audio, even with low res proxies. Any idea what the issue could be?

I had to manually install the graphics card myself and to the best of my knowledge I did it correctly, would this cause the issue?

The Specs look good enough to be up to the task. What kind of footage are you using, project resolution? Did you run some benchmarks on the SSD, and the CPU, GPU to see if it's performing as intended?