I remember the app only synced while on the home screen, but now it seems to regurlarly sync while I am performing reviews, sometimes locking the app for 10-20 seconds. This is pretty inconvenient.

I second this. And even worse when my mobile network is bad, then it will keep trying to establish the connection, but never succeeding.
I'm in Beijing.

Got the same problem too...

Is this happening while it's adding words, or not? We may need to configure the small vs. large sync threshold some more.

Does not seem so. I added a lot of word manually had about 1500 reviews to do when it happened.

I've had it without adding words and immediately after successfully adding words...

FYI, Sent you a separate email that may be related to this as well.

Seems to be random. Seems like a lot more frequent syncing and the syncing is slower than it used to be.

Mine shows the following:

30 seconds of
《Sync: Counting 0/0》
30 seconds of
《Sync: Assembling 0/148》
Only then does the assembling start, and now, rather quickly, it reaches
《Sync: Assembling 148/148》
Then it starts doing the saving, also at a fine pace
《Sync: Saving 138/148 (0.01 MB)》

I am not sure if it has anything to do with adding new words. I have been adding quite a lot of single characters that I have already studied, so it may be connected.