Blender preview in video editor is really slow, barely using my computer's resources I set the memory cache limit to 12gb, I'm running on ssd, I even set the resolution to 5% yet blender is ungodly slow. I'm only getting around 5 fps and blender starts to lag heavily when I start to add effect strips like speeding up the video or adding overlays. The playback is set to No sync.
I know I have a slow system but this is ridiculous. I'm running on a AMD Ryzen 2400g and a Radeon 570 with 16 gigs of ram but I noticed that its only using 13% of the CPU, 235mb of ram and it barely touched the GPU.
Does anyone know how to fix this? I'm editing a 3 min phone video, uncompressed 1080x1920, using the Eevee engine
Actually, there are a couple of things you could test, if they will improve the performance.
Try the Prefetch Frames(Cache) in the Preview Sidebar:

Try the new Disk Cache, which may be fast enough if you point it to save on a SSD:

Another thing to test is to make the project resolution(default 1920x1089) match your footage resolution(1080x1920), by selecting the strip and select this:

Cache handling seems to have improved in 2.91: https://builder.blender.org/download/
Finally you can try to use proxies. This add-on makes the process very simple and fast:
https://github.com/cgvirus/blender-vse-easy-proxy
FYI: Work is being done in this area currently. Ex. yesterday a dev found a way to speed up ffmpeg with 24%: