When I first tried W10, I noticed a distinct lag before the start menu opens. Often, menus would pop under the taskbar, instead of over, being unreadable and unclickable.
I assumed they pushed it to market before it was ready, allowing users to find the bugs to save on testing costs (helping justify why it was free). Only, now, years later, the start menu still lags, and things still open behind other things.
Combined with all the mysterious data it sends to various IP's, I think that when you press Start, those first 300 ms are spent as part of some sort of distributed computing effort.