Did you add an SD memory card for additional storage?
If so, please make sure to use a fast and reputable brand SD from a reputable retailer. A slow, poor quality or counterfeit SD card has a negative impact on the performance of the phone.
Restore of a system backup from another phone or installation of third-party apps from unofficial sources could also be the culprit.
I'm sure the forum will be helpful if you have more specific questions, but you need to perform some basic troubleshooting yourself.