Designer’s role doesn’t end at UX conceptualization, it actually should be on peak from there. One of the most essential parts of design is; performance that plays a vital role in making products addictive and left with good user experience.

Along with formative and summative usability testing there should be dedicated performance testing of each interaction, how fast it responds to interactions and provides feedback as waiting is not something people like to do when trying to complete a task.

Provide system feedback within 400 ms in order to keep user’s attention and to increase productivity. As per Doherty threshold computer should take to respond before the user could get bored.

If there isn’t much that can be done to improve the performance

There should be perception creation that a website is performing faster. One such example from the leading social media app Facebook which shows a presentation of skelton screen when the content is loading. And these blocks are progressively replaced with actual images which increase the perception of speed even if content is loading slowly.

If the system responds more fast then user’s expectation

Change blindness may occur; change that happens too fast may be completely missed and can be difficult for users to comprehend, what happened as the user has not sufficient time for mental processing.