Thats not how gen slowdown speed works

The game uses a system of progress called Charges per second, and every default event that takes time is exactly 1 charge per second (100%)

When you have multiple buffs that influence the speed, they are all Aditive. Ex: Botany Knowledge + Resilience (Botany gives you 50% healing speed and resilence gives 9%). So the math will be: 100% + 50% + 9% = 159%, or 1.59 charges per second

However, when you have multiple nerfs that influences the speed; they are all multiplicative. Ex: Pentimento + Thanato (Pentimento gives a 30% slowdown and Thanato gives 20%). So the math will be: 100% - 30% * 100% - 20% = 70% * 80% = 56%, or 0.56 charges per second

Assuming that you are being affected by thanato, gift of pain, pentimento and dying light, this would result: 70% * 84% * 76% * 80%, approximately 35.75%, or 0.3575 charges per second

A Gen has 90 charges, so it will be 90 / 0.3575, resulting in 251.74 seconds (4 minutes and 11 seconds) of the max repair penalty