Correct... as anyone who's tried modifying a bash script while it it running has found out.As far as I know, no Bourne-style shell has ever tried to read an entire script into memory before executing.
Don't know about that specific brand, but not all USB sticks are created equal. Performance varies based on size and cost. USB 3.0 doesn't buy you anything since the Pi's interfces are USB 2.0. You are subject to USB 2.0 tranfer rates.The USB stick is a Toshiba U364 USB 3.0 32GB TransMemory.