That depends on whether you are pulling up your email on cell data or wifi.
If it is cell data you can reset your network settings by going to settings-> general-> scroll down to reset -> reset network settings
If it is on wifi do the same thing as above but when the phone is resetting unplug the power cord to your router and count to 30 and then plug back in. Make sure you are also next to your router to make sure you are not too far away from the connection to get a fast internet.
Speed of your internet is usually based on the connection you are receiving it from.
Since I have gotten my iPhone 4s email is taking forever.....I had and iPhone 3gs and mail came up much quicker....I think it must be the operating system on the iphone.....it took about 4 minutes for my emails to finally show up...this should not be....iPhone 4s has many mor GB than my old phone and it should be running much quicker but it is not...
Are you trying to open the same email account on both devices in the same location?
Does it also happen when viewing webpages or just opening email?
And if opening the same email are you opening it in the mail app on the phone, a third party app just as a gmail app, or through webmail by using safari?