So mastery increases the hp of my buildings but does it also increases the rate of which my golem repairs my buildings? If a golem with mastery 1.5k heals as much as a golem with mastery 3.5k, the second golem will need alot more time to repair my buildings (more hp potentially lost). Can anyone clarify?

To my knowledge, no. The golem is tougher but not more efficient.
If it was allowed to affect the rate of repair it would also increase the rate of many things on many dif char types.

So Mastery.

If you look at you char weapon you See "Base Damage" and "Base Power Attack Damage"
If you look at you char Alter tab and go over the abilities you can see some like "Base Power" and "Base Armor" Or "Ability Power"

These numbers you can see are the Damage in the game if you have 1000 Mastery.

BUT for dwarfs you have also Base Armor, Mastery increase those too, so the more mastery a dwarf get the stronger his constructs get.

So mastery increases Damage and Healed.

To get the exact number with mastery this is the formula:

The 5 mentions ability/power * Mastery / 1000.

So if your Base Damage = 100 and your Mastery is 1500 1 Left button with no block will give you a number of 150 on your screen.

For the Dwarf Construct it the other way around the number you get is the damage you need to do to the construct to destroy it.

So a higher mastery makes a golem heal more?

If you look at the Golem skill.

"Base Power" and "Base Armor" Or "Ability Power" are increased. Not the speed.

If you want a speed increase you need to works with a second gunner and put 2 Golems.

Golems do not heal a Golem.