Premire Always Lag Playback on a High End Computer i9?!

Make sure your Default Input in the Audio Hardware in the Pref is set to NONE.

It's partly because your GPU is nearly obsolete. All driver versions after version 440 (which introduced CUDA 10.2 drivers) have depreciated CUDA support for GPUs that have Compute Capability version lower than 5.2, meaning that these older GPUs have their CUDA version locked to 10.1 even with newer CUDA versions. Unfortunately, all 700-series GPUs fall into this category of depreciated GPUs.

And the version of NVDEC in Kepler GPUs simply cannot handle 4k resolution well, if at all. The maximum practical limit for these older implementations of NVDEC is 1080/59.94p. Unfortunately, you have an HEDT CPU, which has no integrated graphics processor at all. Therefore, you will not have QuickSync at all to fall back on. As a result, you're stuck between Kepler's lousy NVDEC implementation and software-only decoding.

It is also possible that the GPU has had its VRAM depleted in the middle of playback of 4k material. In this instance, the decoder will be slammed ungracefully into the software-only decoding mode for the remainder of the playback job, with absolutely no indication whatsoever that this is occurring. In other words, even though at 3 GB it is more than Adobe's minimum requirements, it is below Adobe's recommended requirements for the amount of VRAM.

Finally, although your system's CPU is a 10-core/20-thread model, it does not have the clock speed or the IPC (Instructions Per Clock) performance of newer 10-core CPUs. At best it can barely keep pace with today's 8-core/16-thread mainstream-platform CPUs due to the 7900X's clock speed deficit. Too bad the LGA 2066 socket is right now in a very tough position with regards to CPU upgrades: None of those that are currently available, not even most of the i9-10900X-series CPUs, deliver a worthwhile enough performance increase over your current i9-7900X to justify their current street prices.