Doherty Threshold

Studies claim that if the user has to wait (e.g., for a page to load) for more than 400ms, then they start becoming disinterested.

This was the outcome of a study from the 1970s, by Walter Doherty and Ahrvind Thadani—so it's possible that in the modern day, people expect software to be even faster.

Churn

• People will churn from slow experiences.
• This happens in a non-linear fashion. i.e., people will tolerate a small amount of latency, but then drop-off fairly rapidly.

Attention & interest

• Slow interfaces (or loading screens) will lower their attention and interest.

Discomfort & frustration

• In some instances, it can actually be uncomfortable to wait. 
• e.g., if you'd just entered your credit card information to buy event tickets, and the interface was slow. You'd be worrying that your tickets had been sold to someone else.