Diagnosis
Slow or failed live view is almost always about bandwidth—either your home internet upload speed is too slow to send the video stream, or your cellular data connection on your phone is weak when you're away from home.
How to Fix It
First, test your home's upload speed (not download) using a site like speedtest.net on a device connected to your home WiFi. The camera needs a steady upload. SimpliSafe recommends at least 2 Mbps upload per camera, but more is better. If your upload is less than 5 Mbps total, that's likely the bottleneck—you need to talk to your internet provider. Next, check the camera's WiFi signal strength in the app (Device Settings > About Device). If it's poor, that adds to the delay.
If That Didn't Work
The problem could be on your phone's end when you're out. Try loading the live view first when you're connected to your home WiFi. If it loads fast at home but not on cellular data, your mobile connection is the issue. Also, close and restart the SimpliSafe app completely; sometimes the app's cache gets bogged down.
Heads Up
Don't have multiple people trying to view the live stream at the exact same time from different locations. The camera really only handles one live viewer comfortably. If someone else in your family has the app open and is watching, it can cause your attempt to time out or be super choppy.