You know that delay that often occurs when you summon a retainer in a crowded area like Idlylshire or Limsa? That delay occurs because your request for your Retainer is just one among hundreds, just to obtain one retainer's worth of stored items. Multiply that by eight times the data (assuming you have eight retainers), and you might as well grab a book to read while you wait for your Retainers to load in. XD

Just get in the habit of distributing your items among your retainers in a manner you can easily memorize, and you should rarely need to move items from retainer to retainer, or get frustrated summoning one retainer after another trying to find the one who has the item you're after. For example, if you're a glamour enthusiast, store all head, body, handpieces on Retainer A, and all body, legs, and weapons on Retainer B. You'll never need to transfer items between them, and you'll always know which retainer to go to for whatever item you're currently looking for. Hmm, I feel like a wooden sword today - where'd I put my Darklight Macuahuitl? On Retainer B, of course, along with my Zantetsuken, Thundercloud, etc.