VNX5400 Deleted LUN Slow Space reclaim for pool

Hi

I have deleted 4TB LUN from storage pool.

Now it's not showing up in the LUN list.

The "free space" of the pool is being increasing, but VERY slowly. (apprx 50gb per minute.

Is it expected behavior? 

By the way, the LUN was in middle of disabling dedup that tooks forever and have impacted overall system performace. that's the reason i have deleted it.