Basically, what should I sell to make it faster?
useless junk (e.g. tier 1 catalysts and reagents, tin cans, old boots), plus armor pieces that are either early/mid-game or just things you’re not going to use (though expensive pieces you can keep for later or sell on marketplace for more drachma)
should I focus on removing different items or volume of items
If you want it faster probably different items.
I’d imagine loading an entirely new slot in your inventory probably takes more time than simply loading a higher number.
Other than the things already mentioned I would also recommend selling sail patterns, since there are a lot and you are unlikely to ever need them.
Different to start, considering the way that AO’s inventory icon loading works (I forget the specifics but it used to cause tons of lag).
It used to take entire minutes for giant inventories to load because of how many different models were loading, thank goodness we got the option to disable that in settings