Handy But Laggy Rated 3 out of 5 stars

I was specifically looking for something that would give a better oversight of my open tabs, much like how there's a dropdown-menu to see all the tabs in a vertical list in Firefox. Of course that doesn't stay open and also doesn't allow your to organize or anything like that.
So I looked for an add-on like that and found this one. I thought "Great, exactly what I needed.".

Now, let me first of all say that, what I expected was the following:
Reading the description I thought it would leave the tab-buttons as usual, and then you could basically add tabs/pages under every tab. So that when you click a tab, it would expand into a list pretty much like the one that's added on the side in this add-on. But instead, it basically gives you a menu, similar to the Bookmarks and History panes, which then has all the tabs you have open, and you can drag to organize them. Not exactly what I was looking for, but at least it does give me more oversight than having all tabs horizontally in one very long line.

Now, although I like this, it could have some more work on being easier to work with and having even more oversight. Cause in my experience it's still a bit hard to oversee in terms of features and all that. It's still a bit cluttered, I guess is the right description.

But here's the major issue I'm having with this add-on, and I'm not sure why:

It's VERY slow. Let me explain first that I usually have LOT of tabs open, so many I'm not even going to count them. But one of the reasons I got this add-on is because I wanted to clean them up and get them in order. Now, ever since Firefox 18 or 19 or whatever, an update somewhere in late 2012 I think, Firefox became really laggy to begin with, and the more tabs I have open, the heavier it is on the system.
And my computer is fairly fast, I use it for, not only games, but also heavier video-editing and image-processing and audio-software, etc. (yet Firefox is one of the heaviest pieces of software since a particular update).
Now I thought I could be a lot more productive with this add-on, but unfortunately it makes Firefox even heavier.
For example, when starting Firefox with all the tabs open, it would first take about a minute (after the update that made it slower/heavier), now with this add-on activated, it seriously takes a couple of minutes. I literally have to just walk away from the computer to let it load, get a drink or something, and check 4-5 minutes later if it has loaded. Even after closing maybe already 100 pages using this add-on, to see if it would make a difference. And not only the startup takes long, it also lags a lot when I want to organize the tabs in the menu, when I want to drag and move them somewhere, it just pauses a lot (while I still have plenty of memory left).
I'm not sure why this is, most likely because I have so many tabs open, and Tree Style Tab has to like load in every tab or something as well, in combination with Firefox already being slow and laggy for me.

So, although this might partially be my own "fault" and also Firefox being so slow nowadays (Seriously, the fastest browser?), it's clear that Tree Style Tab needs an optimization in performance. I can really tell that Firefox is now even more resource-heavy, cause I even see the CPU-meter run higher than it usually does with regular laggy Firefox. It now varies between 10-20% of the CPU-usage which might have been half of that before using this add-on. And it also still needs some optimizing in terms of clutter and the way it works. Other than that, I think it's a great idea, it's not far from what it should be in my opinion, apart from the technicalities, and it's also great it has so many features. But please, optimize to make it even more functional and productive.

For the rest I would like the option to keep the tabs above the page, because I feel rather handicapped having to go through the menu at times. I'm not sure if the option is there, I could really find it. And if the option is there, it only shows what I mean by how it's kind of "cluttered" in use.
It would be even nicer if the tabs could have like dropdown-menus/trees like I described earlier, but I think it would become a whole other add-on. If it's still possible to add that, it would be great of course.

I would recommend this to anyone, but only with both the pros and cons in mind.

Oh, and please note, I don't use many other add-ons that might interfere (but who knows they do) other than ReadItLater (or Pocket I guess) and... I don't even know... Yeah, Adblock Plus... That's all really. And always the latest version of Firefox, by the way.

This review is for a previous version of the add-on (0.14.2013040601.1-signed).