Sounds like a Time Zone issue. Are the SEPM server, client and SMTP server all in the same Timezone and correctly set for DST, with all relevant MS patches?

The SEPM server and the clients are all set to GMT.
The email i get about Scheduled report: Virus Defs Distribution is ok the time in the email about when the report was generated is the same time i get an email was sent to my inbox.

Its just on the single risk email alerts

If you are in pull mode, the rate that your heartbeat is set at will affect this.
If your hearbeat is set at 30 minutes and ... and the machine's last heartbeat was at 10:15 AM and...
If you get a virus at 10: 30 AM, you are not going to get alerted until at least 10:45 AM.

at least that's my understanding..

edit:
nevermind, I just re-read your question, and I don't think I've provided the correct answer.

There is "Damper" parameter in Notification settings. I guess it prevents your mail box from notifications flooding. By default it is set to "Auto" that if i am right is about 1 hour. Server sends only 1 notification in that period. If there was 2 notifications simultaneously, server sends 1 notification and other only after "Damper" period.
You can set "Damper" to 20 minutes minimum.

If i set it to 20 minutes its 1 hour and 20 mins. So it increases it an extra 20 mins on top of the 1 hour already.