First, please, for clarity's sake, rename that index. Anytime I open a server and see _dta at the start of an index, I almost want to cry.

I would suggest you capture the wait statistics on the process as it's running in order to understand where your slow-down is coming from. I'd think it's either in memory or possibly waiting on I/O within tempdb, but understanding why and where is going to be a big help.

How much memory does the system have? Is tempdb and your database on the same disk? What's the clustered index on? I can see that you have one, but not the definition.

But, at the end of the day, you'll probably have to throw hardware at this problem. You're trying to sort a very large data set. You need memory and lots of fast disks to get it done.