It wouldn't be an issue if this was a single spawn like on your pictures.
I have now, 2 reliable saves from adventurer site, there I didn't zone a thing. 1st is lag free. 2nd is crippling 3(2)-2(1) lag. The lag is also repeatable. I just need to run to east until my 5 companions return to the construction site and vanish in fog. I see 1-2 animals sighted during this run. Then on return I see 1 spawn of new animals, like on your picture or 10 large spawns (like 12 of each) of different animals like entire adventure site was suddenly covered with huge amount of animal spawns.
The cleared lairs also show same mechanics. Cleared lairs like adventure sites spawn huge amount of wild animals' large spawns. Though I didn't see lairs to cause any huge lag yet. Huge lag happens only on adventure sites. To go from 1400(50) fps to 3(2) with loading black screen for adventure site for 30 secs... now that is crippling experience.
Now I will go to file site and then to bug reports with those 2 files. Maybe Toady can figure finally out, what is happening there and what is causing it.
oh my picture is showing off multiple critters being spawn, there' also Daemons being spawn when you move around, but you don't see them as they are underground.
clear lairs save any changes done to them so do camps,
the bug is caused by the fact traveling around spawns critters on top of doing so near a site so the site is constantly being filled with critters from just walking near the edge.
one could avoid this by Fast traveling out of their camp or building
using putnam's handly goku teleportation script as a unit scanner I can show off what causes this bug to happen.
the pause you see in the gif is the game loading and unloading new parts of the map.
also advmode map is alot larger than what is shown on screen so several, Several Miles/kilometers to the left and right are loading and unloading chunks of land and in doing so unloading and loading in units that habitat on those lands.
now imagine doing this process of walking back and fourth 1 embark square away
and here's my solution.. which took awhile and requires dfhack.
and the preventive solution to this problem which requires standing as close as possible to the site you want to visit in the travel menu and possible to do in vanilla.
so long story short it seems fast travel is completely FPS friendly option to traveling, and or Traveling towards sites causes the game to load the whole site then walking away from it unloads the whole site and in doing so saves the units there or uhh forgets to unload the wild animal clutter.