I have 4* ranger hero here who already have level 7 ranger talent.
Now there’s option between atk+ or mana speed+ , and I’m not sure which one worth more seems my hero already have fast mana.
So I’m wondering will the mana speed+ make him to have very fast mana? Just wondering. 
Is the mana speed+ really worthy? Please share if you did your hero’s upgrade for mana boost talent.
The mana speed boost is exactly 2% faster mana generation, which alone, is absolutely worthless, as it won’t charge up in fewer tiles than before.
If you have a 4* mana troop you are using on that hero, there are breakpoints for those, depending on the hero’s mana speed, and for that hero, the breakpoint is one lower:
If the hero in question has fast mana (as you indicate), my initial thought is don’t bother: To get any improvement you need a L29 mana troop (nearly unachievable unless you spend a couple thousand dollars on troop summons) … or if you take this mana boost, you will get the effect with “only” a L23 mana troop (which typically can be achieved in a over a year of playing to get the feeder troops).
The place where this looks better is for an average mana hero – Getting a 4* troop to L17 can be done by farming in a ~year of gameplay farming, and add this boost to get the improvement; instead of charging in 10 tiles, the hero will charge in 9 tiles (three match-3’s)
If you search, there’s a lot of discussion regarding 4* mana troops vs 4* crit troops on the forums, to give a bit more context to my answer, @WaaWaa
I see~ thanks for the explanations~ 
I do have L17 mana troop but it’s not for my ranger hero~ 
guess I’ll not pick the mana boost then~
I don’t have it readily available, but the formula might help OP understand how it works.
Anyone else have formulas?
That’s the ticket!
Thanks