Significant delay to notification emails?

[ember1205]

I have signed up to get alert emails for all of the various 'reasons', and they do [eventually] arrive. It seems like there is an hours-long delay in between the activity that creates the alert (someone posting in a thread I'm watching, for example) and when the email actually arrives. Is there a setting someplace to change this? Every other forum I belong to offers notifications that come through within about a minute.