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.