Email Receive Lag Time

On my Instinct HD, I have two email accounts set up as regular POP3 accounts.  I also have another email account set up as the GMail account.  I can send an email to my GMail account from another computer, and it beeps in on the phone within a minute or two.  When I send emails to either of the two other POP3 accounts, it could take 20 or 30 minutes for me to be notified.  Of course, I can SYNC for either of those accounts and the email will be displayed, but what might be the difference between the email accounts that might account for the difference in email notification times?

Thanks in advance,

HL