I expect it to be slow... but once it boots, runs miner, loads everything needed into RAM... than there should be no slowdown, right?
It's fast! The main time booting is waiting for the hard drive. Sticks just fly.
If you computer is slow after booting, running on a stick won't help much. Linux caches everything so
if it's slow (after booting) you probably has too little RAM and it's swapping. (That will make your stick
last shorter.)
When I last installed linux, it required second partition with the same size as available RAM (I'm not sure about purpose of that).... can this be somehow avoided?
That second partition is probably the swap partition.
I don't know what Ubuntu demands, but Linux works fine without a swap partition if you have enough RAM.
IIRC, you need a swap partition if you want to suspend to disk, e. g. if you have a laptop. As you mining
that's not likely.