Is there a way to get instant emails or notification from Stack Overflow?
3 hours is too late to receive emails.
Why is the instant emails not yet implemented?
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The most likely explanation is simply one of sysadmin considerations. The way things go on Stack Overflow for a post of average popularity is that it gathers 20 comments and 4 answers within an hour, then interest drops down to a trickle. If all of those enable instant notifications, that's several hundreds of emails to be sent. If the notifications are saved up for 3 hours, that's 20 max.
Sending out 10 to 20 times as many emails as they are doing now requires a specialized infrastructure that takes blacklisting, abusive spam reporting etc. more fundamentally into account, not to mention it requires extra servers to process all the mail queues, bounces, relaying etc. I wouldn't be surprised if Jon Skeet's email provider automatically blocked his mailbox if he started getting at least 1 email per minute for all the topics he's ever been active in, and send an automatic blacklist recommendation to DNSBLs on the SO mailservers.
Saving notifications up for a relatively short period saves an awful lot of server resources, and immensely reduces the chance of blacklisting problems.
If you're in a hurry for an answer, just keep the site open. The notifications bar in the top left automatically updates when there's something going on.