All inventory is loaded on every zone. For someone who has items in every bag, it takes 40-50 seconds for all items to load. Imagine starting an ambuscade or HTBF and you do a job ability, but the relic armor that augments your job ability hasn't loaded yet? You have effectively wasted your JA because the inventory hasn't loaded yet. (This is also verifiable, by the small inventory/sword icons spinning in the top right of your screen, when you zone into a new zone). Give it a try, you'll see what I mean.
I play 10-14 jobs, well geared and the lack of inventory space if a huge problem for me too. I think SE needs to do something about the loading times as well as increase storage. Unfortunately, they don't have a lot of options:
1.) Data stored by key items
2.) Data stored by items
3.) Data stored by items with EXT fields (storage slips, augments, etc).
4.) Increase number of containers that can store items.
5.) Make some containers only available in mog house or wardrobe?
6.) Increase packet length from 200 bytes to something ....larger.
SE has options but they all come with pros and cons. Unfortunately (again), SE said in the latest Q&A they they don't even have dedicated engineers for this game.
It makes me very sad as whole, especially since last May, they said they now have 2 dedicated developers....I guess that did not last very long.