and 'a certain size' isn't even a big number. I used to do this sort of stuff with SAP on-premise and it was very fast.... so frustrating for the NS user. Still quick than manual assignment I guess.

are you working with a client script?

I've actually seen the opposite behavior, so would like to know more about your situation. I even recently wrote a script that adds 10,000 lines to a transaction (the max) and it still saves in a reasonable amount of time. In fact, sublist manipulation seems quite speedy. Using sublists is even a performance hack for updating many records quickly (leveraging parent/child) vs updating the corresponding record instances directly individually.