I've downloaded one of the latest nightly from this place:
http://build.koreader.rocks/download/nightly/
Before (with the latest release from 20170821):
KOReader started immediately. The first time I searched in the dictionary took almost a minute (I have three dictionaries installed).
After:
Now it is the opposite: KOReader takes almost a minute to start. First search in dictionary takes no time at all.
If I delete the dictionaries, KOReader starts as before.
I know the Github-guys have worked with the dictionary feature recently.
So is this an issue related to that or is the change intentional?