I've encountered this as well. I'm using Classic Shell which has probably a quarter of a second delay, but I briefly tried StartIsBack and the delay was multiple seconds. It seems to be caused by SearchApp.exe trying to load News and Interests (ShellFeedsUI), but disabling SearchApp entirely makes it worse. I've seen other people with the same issue caused by updates last year, but no word as to if it was ever resolved. With a clean consumer edition ISO, you'll probably have no issues, but I'm using LTSC which never had News and Interests to begin with as far as I know.
