location of media clips and impact on speed

Since my the playback of the clips has been very slow, I experimented with a few different scenarios... but still can't figure out what is the best way to maximize performance.

Currently I'm working with two internal drives (Intel NVMe 960 Evo M.2 SSD and a 7200rpm HDD).

Have my OS and PP installed on the SSD.

Experimented with having all the cache on SSD vs the HDD... couldn't really see a difference.

Having the actual clips on an external HDD was the slowest.

However, yesterday I had an external SSD plugged in and had the project files and the footage on there and everything was WAY faster.

Why is this ?

The internal m.2 is faster than the external one over USB3.

I am getting a new Macbook soon, with an internal SSD and an external SSD.

What would be the best setup in this case ?