RX 580 8GB working really bad in Premiere Pro CC 2019

The reason for such lousy performance with the RX 580 is that OpenCL is, frankly, effectively "depreciated" throughout the entire tech industry. You see, outside of experiments the official OpenCL specification (from the consortium that maintains OpenCL) has not been updated significantly since 2012. That meant that even the Radeon RX 5000-series Navi GPUs (the current generation) are bound by those ancient support rules (your RX 580 is now two full GPU generations old, being of the Polaris architecture which predated the Vega architecture that came in between Polaris and Navi). And Adobe's relative lack of performance and support for OpenCL shows. Slow performance and corrupted renders are two of the biggest problems. In other words, even with the most powerful GPU, OpenCL performance in Premiere Pro is lackluster at best.

Sadly, the only way to break this lackadaisical OpenCL support and performance throughout the software industry for AMD GPUs is for AMD itself to come up with a Windows-compatible proprietary API that's comparable to Nvidia's CUDA to replace OpenCL. AMD, unfortunately, is currently unwilling to do that, so it's stuck with OpenCL in Windows for the foreseeable future. The only other alternative that's available for AMD is to completely omit GPGPU processing support, which would permanently force Premiere Pro into the software-only rendering mode.

And since all Nvidia's lower-end GPUs at the same relative performance level up to and including Pascal are not worth their current street prices for the relative sideways-grade from your current GTX 750 Ti, the only fix would be a brand-new GeForce card of the Turing architecture. That means the GTX 1600 series. And with that first-generation 6-core/12-thread Ryzen that's currently in your system, I would recommend only two GPUs within that GTX 1600 series (depending on your budget): The GTX 1650 SUPER or the non-SUPER GTX 1660. (Nvidia does not offer a GPU that's in the RX 580's price range that has more than 6 GB of VRAM.) Anything else above it is quite overkill (including the GeForce RTX 2060 SUPER, the least expensive Nvidia GPU that's equipped or available with 8 GB of VRAM).