As I've been there, too (originally staring with an "USB stick" to run Qubes OS, I found out that it's way too slow. Then I tried on external USB-connected hardddisk, and it still was slow occasionally. Finally I bought an external USB case for an NVME SSD and a fast NVME SSD.
I can only recommend that: It's *much* more fun using Qubes OS. VMs start in maybe 2-3 seconds instead of 30-40 seconds...
P.S.: I'm using an external drive for Qubes because I have filled my four harddisks with other stuff already, and a fifth harddisk would would have bee an organizational challenge with the desktop case I have.