Not entirely true...the upgrade on the PEKKA will pause/stop, until the Lab finishes.
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What about the queen? An extra wallbreaker would help a lot, I think!
I am actually in a really bad spot with DE upgrades... and as a teacher my free time to devote to this game is about to decline in a big way. I was sort of counting on the drills to bail me out as I try to catch up on my lab upgrades. The only thing I've done right is get my BK to lvl 5.