- 4xx codes: most of the time URL is not found due to broken links, or forbidden.
- 5xx codes: server errors.
- TCP errors: network error.
- Malformed URLs: the URL can not be fetched because of syntax problems.
- DNS resolution failed : the hostname of the URL is unknown
- Connection failed : the web server or the network might be down or too busy
- Request timeout : the web server could not reply before the specified delay (surely because it is too busy).
- Slowest time to first byte (> 250 ms)
- Slowest file loading time (> 250 ms)
- Pages requiring most http requests
- Slowest DOMContent events (> 500 ms)
- Slowest onLoad events (> 2 s)