Here wood delivered by your lumberjacks is processed. Based on its level your sawmill can increase your wood production by up to 25 percent.
The percentage added to your production is based on the gross production of woodcutters. This means the production of your fields without any bonuses.
The level dependent attributes (such as construction costs, generated culture points and overall crop production increase) can be found here.
For the full table of construction times click this link for server with 1x speed and this link for server with 3x speed.