Duration modifiers working strangely

I just tried combining spearfield with Fast forward and Delayed Reaction support gems, and something doesn't work as described.

-Spearfield has base duration of 10 seconds.
-Fast forward alone says and gives 30% less (according to tooltip which display an expected duration of 7 seconds at this point, wich correspond to reality)
-Delayed reaction alone says 70% less, but tooltip display 10 seconds, and test with a chronometer gives about 3 seconds, which is expected with 70% less duration.

Now, when you combine both, tooltip displays 7 seconds, but duration is about 3 seconds.

Now, knowing that i allocated no duration modifiers on my passive tree, what is going on ? How do multiple "less" multipliers apply ?

Additively ? Then i would get 100% less and spearfield should detonate instantly.

or Multiplicatively ? then what should it be ?

Something is off, and the tooltip doesn't help.