Crux and Lendt experienced the largest increases, though they've confirmed some increase everywhere in the outworlds except Rupert's Hole (which they just don't have enough solid data on). Those two are the closest, physically, to Pierogi. It was more of a one-shot increase than a continual climb, though that's a retrospective analysis, since a lot of people didn't notice the increase immediately or weren't able to properly take advantage of it.
The timing is not really clear, because it wasn't immediately obvious, and because people do get better at things without mysterious influences; they're fairly sure it wasn't starting a month before or a month after Pierogi's fall, but there's no "this day is when it happened" to particularly nail down.
There are a number of speculations on the cause; they range from