Q: iphone 6s get slower and some time stuck

Hello and welcome to Apple Support Communities, Engg.farhanmalik. 

If I’m understanding you right, your iPhone 6s has been running slow, freezing, and it has even restarted itself. I know when I buy a new device, I don't expect it to have problems after only a couple of months, so I can certainly see why you would be concerned. I'm happy to help you get to the bottom of what's causing this. 

There are several reasons that you could be experiencing these problems on your new phone. Due to the nature of the issues you're experiencing, we recommend that before you complete any troubleshooting, you first create a backup of your iPhone. Use the support article below to view steps on how to back up:

How to back up your iPhone, iPad, and iPod touch

Since it could be related to low memory on your iPhone, the first thing to check would be your device storage. To do that, open up your Settings app and select General > Storage & iCloud Usage. Under the section titled "Storage", it should show you how much space you have used and how much space you have available to use. If you're running out of Available space on your iPhone, the next step is to delete content to make more room. Keep in mind that you should  only be looking at device storage space and not iCloud storage, which is separate from how much actual space your iPhone has. 

On the Storage & iCloud Usage screen, select Manage Storage. You will see a list of your iPhone apps and you can tap on them to see how much space they use. If you choose to, you can tap Delete App to remove some of your applications that you don't use often. Another easy way to clear out space on your iPhone is to import your photos and videos to your computer and then erase them from your iPhone. Alternatively, you can also reduce the size of your Photo library by turning on iCloud Photo Library and optimizing your storage. Use the following support articles for steps on how to do both of these things:

Import photos and videos from your iPhone, iPad, or iPod touch

iCloud Photo Library

Once you have cleared space, restart your iPhone:

1. Press and hold the Sleep/Wake button until the red slider appears.
2. Drag the slider to turn your device completely off.
3. After the device turns off, press and hold the Sleep/Wake button again until you see the Apple logo.

Restart your iPhone, iPad, or iPod touch

Test out your iPhone after it has been restarted. If you continue to experience the same issues with your device, the next step would be to restore your iPhone using iTunes on a computer. This will help determine if it's software or hardware related. Use the steps in the following support article to restore your iPhone to factory settings:

Use iTunes on your Mac or PC to restore your iPhone, iPad, or iPod to factory settings

After the restore is completed, give yourself time to test out your iPhone before restoring your data back to it. You can find the steps on how to restore your backup to your iPhone by clicking on the "Restore from a backup" link at the bottom of the aforementioned support article. 

Thanks for using Apple Support Communities. 

Take care!