Filling cells by right clicking is slower than in previous versions. In 1.70 you could pretty much fill cells as fast as you could right click. In 1.71 there seems to be some sort of delay built in. This may be an expected addition, however the delay does not seems to be consistent when there are flowing blocks around the source block being selected.
You can demonstrate the delay by stacking three lava source blocks up an then collecting them by right click. There is a pretty significant delay between the collection of each source block into the cell.
It is possible this delay was added intentionally, although I don't see anything in the change log. I really hope not though.