The original timer was a prime factor of the difficulty to solo the activity when these secret missions launched. It took me many attempts to learn how to do so on the Whisper mission, and I was cursing the timer then as well so I can understand the feeling. But it was important to teach this skill for later solo content, and probably the best training for someone who wants to learn how to solo. Near the end of Arrivals when it was about to be removed, I was completing that mission regularly without issue. When Presage dropped a couple seasons later, I could solo it with ease. They doubled the timer here, which is quite generous honestly, although it does lead to some concerns about what they changed. Never got around to soloing Zero Hour so I will be interested to learning that one again. But as with most things in the game, it is usually much better and faster with a team, so it likely will not be an issue with many players, especially with the easy new Fireteam Finder.

It adds pressure and I like it. Interestingly the timer on the normal mode missions is ~40 minutes, I think? 20 is good for those missions.

That was in the original one as well. My guess was to add some difficulty