I update my Nokia Lumia 720 to windows 10. But it works too slow , also it stuck many times. The camera takes much time to captures photo. What will be a solution on it?
Windows 10 hasn't been optimised for certain devices yet, so performance can be poor. Especially on less recent, less powerful devices (Lumia 520, etc).
You can either stick with Windows 10 and see if it improves with updates, this may take a month or two on the Windows Insider programme - or roll back to Windows Phone 8.1.