I've updated my iPhone 6S Plus to the latest software, iOS 11.3. But my phone is now very slow and almost impossible to use! What can I do to fix this? Is there going to be a new update to fix this bug?

You don't need an update. The simplest is a hard reboot/reset of your phone; press and HOLD, at the same time, the home and sleep buttons until the apple logo appears. If that doesn't help, you need to do a factory reset and then a restore. Depending on your apps, this one can be a pain as you may have to reenter many user names and passwords.

Of course, back up BEFORE you attempt anything; but then you regularly backup anyway, don't you?

Saraj im sorry to hear about your issues with your iPhone 6S. I guess when you got your mobile new, it was still using ios 10 right? And so far i remember the 6S and the 6S plus have 2gigs of ram. Now meanwhile the iPhone 8 and iPhone X are out, both of them have 3 gigs of ram and get sold with ios 11. Apple is one of the few mobile sellers who normally have a awesome update policy. Even old devices get modern software. But that’s also a issue. Because older hardware will not preform as fast with ios 11 like it did with ios 10. Like i told you, your iPhone 6s has 2gigs or ram while ios 11 would prefer 3 gigs. I installed like You iOS 11 on my ipad air 2. It was really fast with ios 10. But as soon i installed ios 11 it transformed into a totally sluggish device. Apps take forever to open, animations are slow and the entire experience is just suboptimal. Guess you are facing the same issue. But thanx God i found a solution. I know you don’t want to preform a factory reset. But that’s the only thing that worked with me. I managed to set it up as new device and that gave me back a fast ipad. Maybe this would also help in your situation.

Regards mate

There is an update after iOS 11.3 about which you are complaining. Please check your software again, you should now have or be able to upgrade to iOS 11.3.1

If you already have the new version and the problem has appeared in both 11.3 and 11.3.1 that suggests the iOS software is not the problem, and you might have a rogue or incompatible app dragging down performance. The only way you can check this is a factory reset, then setup again and reload your apps on at a time until you find the offender.

Check your battery health to see if your iPhone is throttling, it could be that. if not maybe try a reset.

I have not noticed any performance degrade on my 6S Plus, now running iOS 11.3.1.

that true same on iPhone 5s

hope apple came with the better solutions very soon !!!