This strat is derived from this thread by Nick: http://www.speedrun.com/gtaiii/thread/di2l3

This mission has always been quite a pain for All Missions runs, because we didn't understand how rampage spawns work. But now we know that there is a variable coefficient that determines the maximum number of peds that can be spawned, and this coefficient changes based on player location (look at Nick's thread for details).

The point is that if you are standing in the areas marked in this image, you will get mostly good spawns and a lot of Triad guys at once: red: very good coefficient (>1.3) blue: good (>1.1) black: very bad (<0.8)

Video:

So my setup is to:

1. start mission, get flamethrower with the replay, then walk to the grill marked with red in the pic, while looking down
2. look up, and turn towards payphone, kill like 6-8 triads in one go
3. go back to the good spawns sidewalk while looking down
4. repeat until you are done

I have had good spawns with this method, and I estimate an approximate 20s timesave on average over not having a setup and/or standing on low coefficient areas.