Google's John Mueller said on Twitter that site splits or merges take longer that one-to-one site moves to adapt and transition in Google's index and ranking.

That means that if you move lets say from domainA.com to domainB.com or http to https and all the pages are redirected one to one with that move, generally, Google is much quicker at picking up those changes compared to other moves. If you move part of your site, change some URLs, move some of the site to another domain or merge some content together into a new or combined URL, those changes generally take Google longer to pick up on.

That means, site splits or merges can take Google a lot longer to index, pick up ranking signals and then rank based on all those new signals.

Site-merges or splits generally take longer than 1:1 site-moves, but there's no fixed amount of time. Reprocessing is on a per-URL basis. — John ☆.o(≧▽≦)o.☆ (@JohnMu) January 15, 2018

This is obvious to most experienced SEOs.

Forum discussion at Twitter.