Available on paid plans from Standard and above on accounts with Simplified Alerting, if you're not migrated yet you will be soon!
Set the timeout value on your site in the check editor when creating an uptime check of any type, HTTP, Ping, DNS or TCP!
There's a slider, it'll allow you to select between 10 milliseconds and 30 seconds. It's great to use, but do not set it too low, because then you'll get alerts for things that are outside your control (a bit of latency is ok really).
It will of course only alert you if this response time has been occurring for the full time you've set "When down, alert after" to, and always at least from 2 separate locations.
For an uptime check (http check) the response time is calculated as the time it takes to perform a HTTP GET to the specified URL, so the response time is calculated in three parts:
- Time to first byte
- Time to receive headers
- Time to load HTML of the site
Skipping dynamic content, most of you will recognize this as basically a cURL request. If you want a response time that is just TTFB (time to first byte) you can use a Ping check as this is almost equivalent.