Minefield support is penalising throwing speed too much

Minefield support gem at level 20 should be reducing your mine throwing speed by 60% and combined with the four additional mines per throw; give you 100% more mines thrown per second.

Using a macro I time how long it takes to throw 15 mines.
With 0 increased throwing speed It takes 3550ms to throw 15 mines without minefield and 2100ms with. At 100% increased speed 1770ms and 1040ms.

This is only a total increase of 70% instead of the expected 100%.
Video example of testing: https://streamable.com/o2smo.

I also notice this too thanks to having 101% throw speed at one point.

(Numbers are 0% increased throw speed)