I have a 6S Plus 128gb 4 weeks old and I get the periodic freezing and once had the "Ending Call" bug where my call was trying to hangup and never did. A reboot fixed the "Ending Call" episode. For periodic freezing and heavy lag I have never reset my phone and will not since it is poor memory management from iOS not a firmware corruption or buggy app issue exactly. I also experience memory bugs in apps that complain of having no internet connection when the cache is full - for example on a popular wallpaper app - flush the cache and it operates correctly again. I believe the issue is memory management not being active enough in iOS 9. Far too many apps are left in memory far too long. A double tap of the Home button shows you what's waiting around for user convenience even after soft reboots. That's not an issue when you are just listening to music, texting, calling and using the calculator a bit but when you are using Safari with 8 open tabs, turning VPNs off and on via script, texting and using 3-4 more apps occasionally during the same 8 hour period it falls apart. I never lag or freeze after a hard reboot or the phone being off or unused awhile. Depending on how things are working behind the scenes I can imagine subpar NAND flash memory chips could cause this but I would imagine I would see lag or freezing when only using one memory intensive app like a game or HD video and that does not happen.
To resolve non-responsive app - aka you can't scroll but the screen stays on based on your touch - clear ram via 10 taps of a system app like iTunes or the App Store. Tap on an icon like the magnifying glass ten times and the screen within the app will go blank for one second while it clears memory. The phone will then work normally.
Sometimes a double tap of the home button and swiping all unused apps up and off the screen will result in the system working normally again after heavy lag.
Sometimes it is faster to hold the power key down until Power Off appears and hold the Home button down until the home screen appears - this is another method of clearing memory.
My last resort is a hard reboot by holding down the Power and Home buttons until the Apple boot logo appears.
I just tried the dictionary reset - General->Reset->Dictionary->Reset Keyboard Dictionary and it made the phone faster for a short period until the App Store and Settings caused things to slow a bit again. I hadn't noticed that happening until watching for delays in opening and closing folders on the home screens and swiping between screens, launching the notifications and Spotlight.
Clearing the memory without a reboot will result in my phone working properly again for days until the next multitask pileup. I don't get any display anomalies like others who have confirmed hardware bugs. This is all occurring with 80gb of 128gb remaining free on my system. I am a power user of sorts and have many IMAP accounts setup in Mail and GMail with years worth of messages downloaded as well as abusing browser tabs. I just came from Android after 9 years of avoiding an iPhone although I am a Mac user, a UNIX and variant user and have iPads and iPhones in the hands of family members so I learned the icon tap RAM tip and others from a hidden trick article. It makes sense to me that this is the kind of issue Apple would have while still perfecting how they want multitasking to work. I'm used to clearing apps in memory on my Android devices to preserve battery life since some apps and games will kill a battery fast left unattended. As of now I'm not upset but I could be easily if I go to have a group call and when I add a party I get that "Ending Call" bug.
For those wanting to go to Android because of this - Good Luck. Spending $800 on an Android phone that gets slow to no updates from carriers unless you use a Nexus device will get old fast. Security bugs are announced regularly and updates not so much. The worst part is watching new phones come out with the current software version and hardware the similar to your current phone but carriers won't release the updates to try to force you to buy a new phone. It's the same program PC makers and Microsoft are rolling out. Apple products still rule resale value too.
