There are other factors that can also lead to screen freezes and crashes. Double check the following conditions on your iPhone 6s Plus:
1) Double check to ensure that you have ample free storage space available on your iPhone. Open Settings ➡️ General ➡️ About ➡️ Available: Here you will see the available storage space on your iPhone. You should maintain at least 15GB or more free storage space for the iPhone’s efficient operation.
AND / OR
2) You may have a poor battery condition which cannot provide sufficient power for peak performance. What is the Battery Health of your iPhone? Open Settings ➡️ Battery ➡️ Battery Health and let us know which percentage you see for Maximum Capacity. Also note what is written under Peak Performance. If your battery's Maximum Capacity shows a value of 80% or less, then it has gone through its lifecycle and therefore should be replaced, according to Apple.
Axel F.