May someone share thought about this.
I've recently saw this folder when i was messing about in %appdata% flock browser.
And from there, the memory from just flock's folder is just tremendous. (128mb)
Checked all parts and found out it was the lucene folder which houses many files. I dare not to mess with the content.

And recently, flock was just getting too laggy, almost unbearable (about 1/2 min for it just to show up)
So i backed up my cookies and personals.
I did a complete clean install. And it runs smooth now, just as it we're a firefox 3. (i'm using Flock 2.0b2)

FYI, it seems that whenever i add some addons, the lucene folder will increase in size as well. (As if it was a recycle bin of some sorts)

Let's discuss shall we.