outlook email is slow coming until I stop and restart

I have been using outlook 6 for at least 7 years , & something recently has me intrigued to submit my question here:
When I hit 'send receive' to activate the mail coming in - if I have a few dozen or more emails coming in they usually run to the 100% in about 10 to 25 seconds .. and I am fully aware if there is a large attachment will take much longer., but as I say recently the emails are coming in at 1%.... 2%..... 3%...etc.. and after half a minute only at about 7 % .. "but" if I hit stop and wait few seconds and then hit send receive again they come in at the speed they are accustomed to ..

Ideas ?