Back in 2010 Foviance ran an EEG study  which showed that participants had to concentrate 50% harder when websites were slow, as measured by alpha waves.
In 2011 Harris International conducted a study for Tealeaf  which found that American respondents got more frustrated by failed mobile transactions (58%) than being stuck in traffic (56%). 35% of respondents had cursed at their phone, screamed at it or even thrown it in anger.
In 2014 Radware conducted an EEG study  which showed that a 500ms delay resulted in up to 26% increase in stress levels, as measured by alpha waves.
In 2018 Philip Tellis showed that pages which were not interactive soon after they rendered (30% after they were visually ready) triggered the greatest number of 'rage clicks' from users . Rage clicks indicate that user is frustrated that a page is not responding quickly, causing them to click or tap on an element repeatedly in quick succession.
More recently in 2020 Cyber-Duck tracked users' blood pressure whilst navigating sites . The results showed that slow pages caused the greatest increase in systolic blood pressure (an average 21% increase) when compared with other irritations such as popups and auto-playing music.
It's clear that slow pages increase stress, and we know that stress is not the response we are hoping to induce in our visitors! Delivering fast experiences will reduce visitor stress and might even create user delight.
You can track proxies of stress on your pages through metrics like rage clicks, but ultimately the outcome of slow pages will be lower traffic, shorter sessions and poorer business results. Core Web Vitals are the best proxies we have for user experience on the web, so optimising for these should reduce visitor stress and lead to improved business metrics.
- [PDF] Web Stress. Foviance. 2010. (back to text)
- [PDF] A report on the Mobile Customer Experience. IBM. 2011. (back to text)
- Mobile Web Stress: Understanding the Neurological Impact of Poor Performance - Tammy Everts. 2014. (back to text)
- UX & Performance: Metrics that Matter (slide 16). Philip Tellis. 2018. (back to text)
- Blood pressure study: Which website issue cause users the most stress? Net Imperative. 2020. (back to text)