Why does it take so long for my pop3 Yahoo mail to be sent to my Hotmail inbox?

I am using winows XP and have POP3 also for MS Outlook 2002 and Ourlook Express. New email appears in Yahoo Mail and is immediarely available in both these programmes but I usually have to wait 10  -15 minutes and sometimes longer ,for the same emails to be available in my Hotmail inbox.