Every millisecond counts when a webpage loads in a browser window — yet nearly 83% of websites have issues that slow them down, according to a study released Wednesday.

SEMrush performed site audits and created performance reports by analyzing 150,000 random websites on desktop and mobile to evaluate their performance, collecting the data using a propriety tool. The goal was to determine how many websites are corrupted by issues that affect the time it takes for their website pages to load.

The findings identify the nine most common issues that influence performance and are sorted into severity from most to less critical. They also show the frequency of the mistakes that are made.

Google uses the page speed as a ranking factor in search engine page results. The time it takes to load a page can seriously test the patience of site visitors. A 100-millisecond delay in website load time can hurt conversion rates by 7%, and a two-second delay increases bounce rates by 103%, according to the SEMrush study, citing data from Akamai.

The analysis also identified that 44% of websites had at least one critical issue. In fact 68% of websites had issues with un-minified JavaScript and CSS files, 42% experienced slow page HTML load speeds, 43% had uncompressed JavaScript and CSS files, and nearly 25.5% had uncached JavaScript and CSS files.

The less critical mistakes include nearly 17% had uncompressed pages, 7% had three or more URLs or loop, 2% had too large JavaScript and CSS total size files, and about 1% each had too many JavaScript and CSS files, and large HTML page sizes.

The analysis suggests cleaning up redirects, trimming down page sizes, and boosting server performance.