Interesting... but I'm not surprised that Bitcoin has the highest usage. I would assume that the amount of data (note: not just the number of blocks) being processed on the Bitcoin blockchain is quite a bit larger than the other private chains.

Still, running on a Samsung SSD... even an external one, I would have thought that it should be easy enough for the system to keep up, given that I have run multiple chains on an HDD.

I'm sure that running it one at a time will stop any issues, but could be annoying depending on how long it's been since that chain was synced and how long you have to wait for it to sync up.