1) The slow motion on your Lumia 640XL will never be as good as the ones in iPhones or other Android phones. This is because the 640XL can't capture 120FPS videos (the minimum FPS to make the slowmotion effect look smooth), similar to how my previous Lumia 830 works. Max it record is at 60FPS, and even editing the said video to be slow-mo looks a lot awful compared to slow-mo videos of a clip captured in 120FPS.
You can also blame the software for not performing well either. Even for my Lumia 950, I can capture 120FPS videos but only capped at 720p. Resulting slow motion video is not as good as the ones found in other platforms.
2) Try checking if your Living Images is turned off under the camera setting. This can be accessed when you are using your camera by pressing the gear Icon. There should be a tick box there to enable this option.
3) Since you just updated from WP8.1, some of your WP8.1 settings are still being carried over to W10M, and these causes some 'version mismatch' (something of the sort) and thus causing some issues within the OS, which translates to performance issues. My advice is to perform a complete hard reset WITHOUT restoring from a back up. Let me write a quick guide for you:
Step 1: Manually back up ALL of your stuff from your phone such as photos, musics, documents, and whatnot (also, make a list of the apps you currently have in your phone) and store them somewhere temporarily i.e. store in your computer. Sadly, this will definitely erase all of your game saves. I do not know if it matters to you or not, but personally, I prefer my phone to work smoothly rather than suffer through the bad-performance all for the sake of a game.
Step 2: Perform a hard reset. Go to Settings > System > About > Reset Your Phone, and follow the pop-up procedure. This will reset your phone as if it's brand new, while still retaining the current OS version installed in your phone (in this case, the W10M OS. It will not reset back to WP8.1 unless you use WDRT, which is a totally different thing.).
Note: Try to plug in your phone, and make sure that when you reset, it has AT LEAST 50% battery left. Some users have reported that if you don't have enough battery and your phone dies out in the middle of the hard reset procedure, it may be bricked. YOU HAVE BEEN WARNED.
Step 3: After the reset is complete, you should be greeted by something similar to when you first turned on your phone: the Out of Box Experience (OBE for short), where you set up your phone with your Microsoft account and whatnot.
Step 4: IMPORTANT When the phone asks if you would like to restore from a back up, choose No. Backing up may bring back some of the problems that caused your phone to perform terribly before the reset. Restoring your reset-ed phone with this renders all your efforts in vain. Just, don't restore.
Step 5: Let your phone update all the apps. This will take a long time depending on your region as well as your internet speed. You can still use your phone, but it will be a bit laggy (it is updating like 40++ apps), so for me, I'd rather leave it alone until it completes whatever it is that needs to be updated.
Note: I'm talking about app updates via the Store, not the one from OBE, where it says "Updating/Installing apps. This will take a few moments" or something of the sort.
Step 5: After everything's done, it's finally time to re-install any apps that you need. Remember when I told you to list out the apps that you use before the reset? Here's why you need that list.
Step 6: If everything performs well, enjoy! If not, then you should either upgrade to a new phone (if you can afford it) or just be patient. All these steps I listed, I've tried them numerous of times with my Lumia 620, 830, and my current 950, and my phone performs very very well after the reset. But as far as everything goes, your mileage may vary (YMMV).
Have fun!
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