Although it doesn't show it in the HUD, Leader is stackable.
Leader also stacks with Template:IconLink, Template:IconLink, Template:IconLink, and Template:IconLink.
These Perks stack additively, so for example: two Tier III Leaders is 18% increased action speed, three Leaders is 27% increase, and so on.
Since Leader changes percentages, the amount of seconds it speeds up an interaction by is relative to how many Survivors are working on the interaction.
# of Survivors within 8 metres with tier III Leader
Time to repair a Generator
2 Survivors
1 Survivor
40.44 seconds
2 Survivors
2 Survivors
36.44 seconds
2 Survivors
3 Survivors
32.44 seconds
2 Survivors
4 Survivors
28.45 seconds
3 Survivors
1 Survivor
30.33 seconds
3 Survivors
2 Survivors
27.33 seconds
3 Survivors
3 Survivors
24.33 seconds
3 Survivors
4 Survivors
21.33 seconds
4 Survivors
1 Survivor
25.99 seconds
4 Survivors
2 Survivors
23.42 seconds
4 Survivors
3 Survivors
20.85 seconds
4 Survivors
4 Survivors
18.28 seconds
The wording on Leader is slightly misleading, for example, if 4 Survivors are working on a Generator and one of them has Leader, it does not increase speed by 9% per each survivor. It will increase the base speed by 9%, and more Survivors working on a generator makes the base amount of time to repair smaller.
This is without consideration of Toolboxes or Prove Thyself, so one could potentially complete a Generator even faster.
Effects on Healing
# of Survivors healing
# of Survivors within 8 metres with tier III Leader
Time to complete heal
1 Survivor
0 Survivors
12 seconds
1 Survivor
1 Survivor
10.92 seconds
1 Survivor
2 Survivors
9.84 seconds
1 Survivor
3 Survivors
8.76 seconds
1 Survivor
4 Survivors
7.68 seconds
2 Survivors
0 Survivors
6 seconds
2 Survivors
1 Survivor
5.46 seconds
2 Survivors
2 Survivors
4.92 seconds
2 Survivors
3 Survivors
4.38 seconds
2 Survivors
4 Survivors
3.84 seconds
This is without consideration of Med-Kits, Botany Knowledge, or We'll Make It, so one could potentially heal even faster.
Trivia
Leader also affects the following interaction speeds: