That 5% haste buff is huge...not just for you, but any caster and especially your healer. There's no reason NOT to have 5% haste. If you're class stacking in a bad way, someone needs to change it up.
That said, I'll try to visualize the effect of Haste on DoTs. Let's work with Corruption because it's the easiest (ticks every 2 seconds for 16 seconds). **NOTE THESE ARE NOT THE ACTUAL CALCULATIONS, THIS IS FOR SIMPLICITY ON EXPLANATION**
With 0% Haste, your tick interval would be 2 seconds. Take 16 sec / 2sec/tick = 8 ticks
With 5% Haste, your tick interval would be 1.904 seconds. Take 16 / 1.904/tick = 8.4 ticks
With 10% Haste, your tick interval would be 1.818 seconds. Take 16 / 1.818/tick = 8.8 ticks
With 15% Haste, your tick interval would be 1.739 seconds. Take 16 / 1.739/tick = 9.2 ticks
Notice that at 15% Haste, we've gained a tick of Corruption. It happened somewhere between 10% and 15%. To reverse calculate it, we can say we want 9 ticks in 16 seconds so our interval needs to be 16/9 = 1.778 seconds or 12.5% Haste. This 12.5% Haste would be reported as a "Haste Threshold" because this is the value where you would gain an additional tick inside the same DoT duration.
Notice that this would only happen if you let the DoT run its entire course. For example, at 12.5% haste, you would have the following tick times...
t=0, time = 0
t=1, time = 1.778
t=2, time = 3.556
t=3, time = 5.333
t=4, time = 7.111
t=5, time = 8.889
t=6, time = 10.667
t=7, time = 12.444
t=8, time = 14.222
t=9, time = 16.000
With 12.5% Haste, you would speed up your tick speed so that you would get 8 ticks at 14.2 seconds compared to 8 ticks in 16 seconds with 0% Haste. These thresholds are important to other DoT classes because they let their DoTs run their course with high amounts of Haste, like Shadow Priests, so they squeeze out as many ticks of damage per execute time as possible. This increases their damage per second, affectionately referred to as DPS. However, refreshing a DoT before its last tick is frowned upon for these classes because it lowers their effective Damage Per Execute Time, affectionately referred to as DPET.
However, with Warlocks' Pandemic ability and the fact that a majority of our damage comes through monitoring procs and updating our DoTs, we don't rely on thresholds. Let's say at t=5 Light of the Cosmos and our Tailoring proc happens. Because of this, we would refresh our DoT. With Pandemic, at t=5, we would refresh Corruption right before a tick (8.8889 ticks, which means 8 ticks have happened and a 9th one is really soon) and empower that DoT. Because of Pandemic, we now have a duration of 11 (the remainder of our Corruption time) + 16 seconds. However, Pandemic says that the remaining time added can be up to 50% of the max duration, or 8 seconds. Since the maximum amount of time able to be added here would be 24 seconds, 11+16 becomes 11+13 with a new duration of 24 seconds but the same tick speed. So you can see that you never really achieved your 'extra tick' from the original cast because we updated it, but we will see 2 additional ticks during this new 24 second duration because of Haste effects.
Because Mastery empowers the damage of each tick, Haste and Mastery work together to increase damage multplicatively. You don't really worry about thresholds as Affliction because your Haste will cause increased DoT speed passively while Mastery makes them hit harder. Modeling these values against each other, we find the maximum DPS to happen when Haste = Mastery.
Hopefully this helps you see how thresholds aren't too important for Affliction to focus on.