I too have noticed increased sluggish behavior. The truth seems that Apple has just switch to a different type of fundamental operating system management protocol. This protocol was designed for the new iPhone 7 and up and although it provides better battery life, it manages memory differently to help maximize the ram that you have. Remember not to confuse storage space (128gb) with ram used to operate the phone. They are different and despite ample storage space, the brain must still manage this. Ultimately, there are some rules to follow:
1. The newest operating system will always work best on the newest phone. Don't ever buy a phone anticipating upgrades or changes
2. Certain new features may not work as well on older phones and should be turned off unless you absolutely need them. Mail fetch is one and turning on reduced motion is another.
3. Never spend more on a phone than you can stand losing in 1-2 yrs. the hardware may last 3+ years but the user experience will decline as technology advances.
Hope this helps....
iPhone 6s