Inventory Space and Data transfer limitations

[Niwashi]

When you do world visit or you go in a instance, they need to move 175 slots of regular inventory + ... [ etc. ]

The main issue isn't when you're moving to a different world or instance server. You do that only occasionally, so the servers are only dealing with it for a few characters at a time.

The bigger issue is backups. Every few seconds that you're logged in, all volatile data about your character --which includes everything in your main inventory and armory chest-- gets copied to SE's backup servers. Multiply that by every character that's online, and it's a lot of data to transfer every couple seconds or so.

That's why systems like retainer inventory are easier on the servers than your main inventory is. Since your retainers' inventory can only change when you summon them, the game can trust that the previous backup of it is still current so long as you haven't summoned them since then. That means there's a lot fewer times that that data needs to be backed up.

(It's likely that the same holds true of saddlebags, that they're only backed up if you've opened them since the last backup. At least that'd be the logical way to program it. But the only time I've heard about it directly from SE was an interview from years ago, before saddlebags were added to the game.)