With the limited info you have provided us, we would just be guessing how to solve your problem. Which navcore version is in the device? Some navcores can cause the device to lag. Too many POI being displayed on the map will make it lag considerably.
The last time I came across a device that lagged very much, I did a complete backup first, then did a clean install using the latest official navcore from Home, followed by installing & activating the map and patching the navcore. The device then ran as good as new. Step-by-step instructions on how to do this can be followed in the clean install tutorial below:
TUTORIAL: Clean Install - How to install the latest map and navcore from scratch!