Kill One Billion Imps

For GZDoom, tested in 4.11
no difficulty settings
How many monsters is too many? A thousand? Ten thousand? Inspired by this topic, here is a map with one billion imps for you to kill. Kill them all to beat the map! Contains advanced functions to prevent your computer from catching on fire, hopefully.
Of course not everyone is hardcore enough to run around and spam BFG shots for 3 months of playtime, so there are also maps with only 20 thousand, 100 thousand, and 1 million imps.
Note that due to budgetary restrictions, the 1 million and 1 billion imp maps haven't been fully tested. If you encounter bugs, a complete playthrough video will surely help in diagnosing the problem.