Yeah this normally occurs (or at least for me) when you get a new wave of dwarves come to your establishment. It seems that the processing of each dwarfs' actions are shared with the new wave, in appose to creating new threads for them.
Fix: Save your game, close application and reopen. This seemed to reallocate threads to each dwarf and smoothed out responsiveness quite effectively.
Hope this helps, and good luck!
chazara90
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