The delay will occur with a non-bootable card, too. Format the card on your computer (file system not FAT32 or EXFAT), format in cam after, boot up -> delay after sleep still there.
Problem is ...
5D3's bootflag can not - at time of writing - be removed in a safe way. And if the bootflag is set, there will be a delay leaving sleep mode.
On other cams you might just use ML for some time, even on a rented one, reset bootflag after and all traces/startup issues will be gone. -> No problems selling the cam or dealing with renting companies.
If your cam has the bootflag enabled and you have no issue with delay ... well, good riddle.