When we've looked into this issue in the past it's usually due to server related issues (slow server), the device or network (mobile device, poor connection), or some sort of customisation made to the templates (ads or other custom JS inserted).
Disabling add-ons and testing in a default style will rule out the latter.
If it happens from a hard wired (ethernet) computer then that should rule out the device and network, which would likely just leave the server.
