Slowdown hypothesis:
The growing mass of glowing green auras is the main problem.
The green aura of zombies is a visually agreeable indicator of an active cure effect.
However, hundreds of (stacked) glowing effects at the same time can't be good for the game's performance.
Can you use some better way of handling this visual indicator for hordes?
Maybe a suitably composed glow effect for the entire group instead of individual effects? (You can not really target individuals in the center of a horde … so a less specific indicator should suffice here.)
For isolated zombies and small groups it should not be that much of a problem and those also benefit more from having an individual indicator.
Test run #1: "Lots of glow – lots of slow"
- Strategy: Play the first few minutes trying to cure a growing horde without actuallly getting any cured people to the rescue places. You can still use the Elite Zombies for some upgrades.
- This accumulates a large mass of zombies and (temporarily) cured really fast while keeping the onslaught reasonably managable.
- There are loads of the glowing green auras since the closely grouped zombies keep infecting each other and turning the cured people into zombies again.
- Result: The slowdown is noticable relatively soon after starting and gets worse quite fast. The sound becomes distorted.
- Stacks of the advanced aura (super spreader) might make the slowdown even worse, but that was not part of this test.
After about 07:00 I led the horde to the shelter. This resulted in hundreds of cured people disappearing (and lots of level ups) in a really short time, but this did not improve the performance much.
The screenshot on the left shows the horde shortly before this "rescue" happened.
(The screenshot on the right shows the end of test run #2.)
Test run #2: "No glow – no slow"
- Strategy: Do not use cure – not at all (!) … just try to survive as long as possible while the horde grows and gets buffed. :-(
- The spawn rate seems similar to test run #1, so this should accumulate beings on the screen at a similar rate.
- However, there are obviously no glowing green auras apart from the safe zone … EDIT: … and the highlighted border of your current cure target (as seen in the screenshot).
- Result: There does not seem to be any slowdown nor any sound distortion.
Without any non-zombie upgrades nor (temporarily) cured people to occupy the horde this is quite difficult. Almost made it to 04:00 …
Conclusion
While this comparison is no irrefutable proof of the hypothesis, it should at least indicate a very promising "point of attack".