I put away the pub/Tom O'Flanagan's, and lag in my town pretty much disappeared. Except over Squidport. Then noticed it got especially slow when I was on Squidport right over Kang. Put Kang in storage, and no lag anymore. Brought Kang back out, set the Victorian spaceship on Squidport to draw him/her there with the Evil Chortle task, and Squidport got really slow again.
Sorry, Tom and Kang. If you're gonna cause me grief, you're gonna stay in the box.
I have lots of neighbors with lots of decoration, but only a few lag. But haven't paid attention to see whether all my laggy neighbors have lots of decorations.
I keep Brandine , Hank, and Arnie outdoors and untasked. The game assigns them random outdoor tasks for visitors, and invariably from my feeders I see Brandine and Hank firing weapons and Arnie up in the sky.