The Construction Research Item states that:
"Every single upgrade increases constructing speed of buildings and fortified units by 10%"
Whilst I would assume this means that:
Lvl 1 Construction = 90% of base time.
Lvl 4 Construction = 60% of base time.
Lvl 8 Construction = 20% of base time.
Lvl 10 Construction = 0% of base time.
However, it appears to be a 10% reduction multiplied at each level. As such, the following provides that actual time bonus / reductions.
Lvl 1 Construction = 90% of base time.
Lvl 2 Construction = 81% of base time.
Lvl 3 Construction = 73% of base time.
Lvl 4 Construction = 66% of base time.
Lvl 5 Construction = 59% of base time.
Lvl 6 Construction = 53% of base time.
Lvl 7 Construction = 48% of base time.
Lvl 8 Construction = 43% of base time.
Lvl 9 Construction = 39% of base time.
Lvl 10 Construction = 35% of base time.
Examples
Lvl 4 upgrade of Ironmine:
- Base time = 12min, 0sec = 720sec.
- Lvl 5 Construction bonus = 59%
- Reduced time = 720sec x 59% = 425sec or 7min 5sec



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