When you add two burns together, it averages the duration and adds the total damage. So a 10 turn burn for 20 damage per tick is 200 total damage, and a 6 turn burn for 100 damage per tick is 600 damage, which adds to 200+600=800 total damage and averages out to a (10+6)/2=8 turn duration, for 100 damage per turn for 8 turns.
Eternal Suffering is ignoring total damage and just extending whatever burn you have, which effectively multiplies the total damage by (burn duration + 3)/(burn duration) which then gets factored into the pertick damage the next time you apply a burn, so yeah, it'll multiply to a large amount after repeated procs. The best way to accelerate the process is to just do what you're doing. Inflict lots of shortduration burns to bring the burn duration down while still keeping the damage, and to get as many Eternal Suffering procs as possible. Destroyer and Abduction get you 5 hits right off the bat, and of course if you're Shalore then you have the global speed bonus too.
Incidentally, 100 turns is also how long it takes you to apply 100% damage reduction with the Oppression Tree. Doombringers are really strong in theoretical 100+ turn fights, although in practice most fights are over by that time anyway.
