An odd thing occurred during a play through - no plans to fix this, since it's just how the game works and we do want to keep the chunk removal - clean up of the map. This clean up greatly reduces lag. Someone who just strolls around the map exposing tons of biters will certainly cause lag, but the regrowth will cover them up again restoring some performance.
But this requires some understanding of how Regrowth could impact your base:
I cleared an area way out - lots of clear areas with a long line of laser turrets to wall off that area. The regrowth even triggered and blacked out an area inside my wall - but fairly close to my wall. Instantly those black areas started attacking my turrets. I walked my spidertrons there and sure enough, full biter bases had regrown. Well you really need to remember to put down radars on those walls. This will keep regrowth from claiming and restoring those chunks to it's state prior to your clearing it.
So why does this happen? There are two surfaces: nauvis and oarc. Oarc is the main surface that you work on that is overlaid on top of nauvis. All of the biters live in nauvis. What is super cool about this is that resetting a player, or a player leaving early removes the oarc layer, restoring what was underneath. Any biter bases that existed prior are immediately returned. Oarc removes and massages the area to reduce the difficulty when spawning in areas that would normally be huge nests. Regrowth returns an oarc area to the original nauvis, full of biters. Oarc code is quite amazing.
Normally this won't be recognized until late game when you move outside the pollution zone to build defenses. Regrowth does not reclaim any area covered with pollution. Oarc code slowly builds a table to define areas that have structures and owned by the player, the starting area and a large area around the start, anywhere covered by radar, chunks with the player in it, or vehicles in it (as of 4.1.9). So cover those areas with radars and regrowth will not repopulate your map with biters.
I timed the regrowth code and it takes less than 100ms (1/10 of a second) - all dependant upon the number of chunks and your processor. So why is there a 30 second warning? Well Oarcinae told me that if someone is in the middle of fighting and there is a pause, this could cause someone to die, so he wants to warn you, so you can get to a safe area for the slight pause.