When I first set up my current gaming PC, I had Kubuntu running from a 1Gb external SSD, just to check all the hardware was good before wiping the main internal SSD.

Used it that way for weeks before figuring that I needed to get around to setting it up on the internal drive. At no point did it feel like a problem. Games were running from a 2Tb HDD, and were playing just fine.

Also had it installed on a 64Gb thumb drive, so I could boot some of the Windows machines at work into Kubuntu for hardware testing.

It really is extraordinary how flexible Linux can be.