While its a good starting point, I dont think we can point at only that as a main cause for faster performance,

if one of those 394 packages includes react then we are at 2839 + 394

a faster compiler/transpiler may be a reason.

Maybe there are other reasons as well

1. SSR is always faster as a lot of the parsing is done on the server which makes time to paint faster

2. next is very blog oriented which means its puts render perfromance as a the critical feature