This is how "Building speed +" bonus works:
basic building time: 10 hours 34 mintes (600 + 34 = 634 minutes)
building speed bonus: 30 %
real building time = basic building time / ((100 + building speed bonus) / 100)
Real building time = 634 / ((100 + 30) / 100)
Real building time = 634 / (130 / 100)
Real building time = 634 / 1,3
Real building time = 487,7 minutes = 8 hours and ca 8 minutes.
Many players understand this attribute the wrong way:
The first thing you have to realize is that the name of the attribute is "plus building speed" and NOT "minus building time". So, you just cannot subtract 30 % from the basic building time to get the result.
You are building a house. If you lay 100 bricks a day, you will finish it in 2 years.
How long will it take with + 100 % speed bonus? (Double speed, 200 bricks a day....)
Will the house be built instantly, in zero time? Of course not, the needed time is halved, you will need 1 year to finish it.
It surely HAS a point to increase this attribute beyond 100 %.
The same logic applies on other "speed" attributes (move speed, recruit speed, magic cloud fading speed.. )
The limit for building cost is 50%
I hope this was helpful. Ask, if you have more questions. 🙂