It was optimizable - if you had a proper Kermit client that did all the stuff ZModem did, like sliding ACK windows and larger packet sizes. After we had error-correcting modem protocols, line noise was much less of an issue.

But most Kermit protocols in terminal software implemented only the most basic version of the protocol and didn't support all the options that you needed to set.

Not sure about resuming, which was AFAICT only a ZModem thing (and the best part of it).