Since I have to study a lot of new vocab every week, so I like to add all new words quickly to make sure all of them get added on time and I'll already have revised them a couple days.

Recently though, it doesn't seem to work as well. Instead of just adding new words periodically no matter how many items are already due, I'll get prompted for half a dozen completely new items in a row.
It made me turn it off and just normal speed and the plus button now.

I'm confused though, did you change something there? Because until very recently, the speed of the fast adding worked very well for me.

Hmm; we haven't changed anything with that recently, no. Do you have an estimate of at what percentage readiness (the "120% due", etc., indicator in the prompt) you would typically be practicing at, where you were getting words at a good rate? It should add a lot if you're getting below 100% (empty blue bar) on the fast setting, but if it's doing it before then, then that's a problem.

I'll try it out again and describe exactly how it goes.

When I finally turned fast of the last time there were still about 100 items due and instead of just adding times now and then instead of not at all at this many items due I was prompted with 5 or 6 new items in a row.