Brad Dickason:

… teams consistently overlook speed. Instead, they add more features (which ironically make things slower). Products bloat over time and performance goes downhill.

New features might help your users accomplish something extra in your product. Latency stops your users from doing the job they already hire your product for.

Slow ui acts like tiny papercuts. Every time we have to wait, we get impatient, frustrated, and lose our flow.