Will be fixed in the next version.

Unfortunately, not at this time. It's something that bothers me. Changing it is on hold until the new config system is in place. Which I'm not sure when I'll have time to finish.

It's not the categories that use the memory it's the storing of the inventory and bank contents. There's really no way around this. Memory honestly isn't a huge concern. TBag uses a hunk of memory but it has to store a lot of info. So that's just the way it is. The use of the memory doesn't equate to bad performance. Memory does not equal CPU time.

I'm not 100% sure how Optionless(house) does CPU/sec but I'm pretty sure it's just CPU time divided by the time since the profiling addon stuff was reset. If you didn't reset after loading in I'd expect a fair amount of cpu time because TBag has to do some setup. But I'm sitting here and running Resource Tools and not really seeing much in the way of CPU time while tbag is closed.

Just a note that I haven't had much time to work on TBag lately but it should work fine on 3.2. I'll get around to an update here in the next few days. It takes me a bit to get data together for items and what not after a major patch anyway.

You can do this already. Though the steps to do it are probably pretty much undocumented.

Make a new category
1) /tbag config
2) Scroll to the bottom and click Add new Cat
3) You'll see two lines with just UNKNOWN. On the next to last one put in whatever category name you want.
4) Click Assign cats (you'll see something like TBag: Unassigned category YOURCAT has been assigned to slot 1).
5) Click close

Assign your new category to your desired bar
1) Open up your inventory window and click on the gear icon (to go into edit mode)
2) Right click on the box labeled 1.
3) Click on Move: YOURCAT
4) Left click on the bar you wish to put YOURCAT on.

Assign an item to your new category
1) While in edit mode (click gear icon if you're not already but you would be if you're following along) right click on an item.
2) Mouse over Assign item to category and then Categories within bar N where N is the bar you assigned the category you wanted to use to.
3) Click on your category.

Repeat the item assignment for every item you want to move.

For the advanced user you can write your own rules in /tbag config instead of using the individual item assignment, but this is probably easier for one offs.

Keep in mind that if you want to pickup sorting changes from the default rules when you do /tbag resetsorts you'll lose the specific item assignments and will have to do them again, though your custom created category will continue to exist and be assigned to the bar even though it won't show under the rules. I realize this is inconvenient, eventually I'll get the new config UI done and we won't have this issue anymore.

/tinv config

Make sure Number of Horizontal Bars is set to a value that evenly divides 32 or however many bars you are actually using. 1,2,4,8 are generally safe values.