"Your phone is too slow" Message that have been shown after every few minutes
Try to stop an application that use so much memory in the background by doing :
1. Go to your phone Settings
2. Select "Apps"
3. Select the tab "Running"
4. Click on an app that you don't use it or have a high memory consumption
5. Click stop
Hope this works.
If this method doesn't work, you might need to factory reset your phone.
That's my suggestion