I would be very careful upgrading to ios 11 immediately on older devices. I'm almost certain that it will take months before subsequent updates are optimized for older devices. With all the new features that rely on the new iphone hardware, ios 11 with it's development timeline will likely only be stable on the new hardware.
Wait until people give feedback and reviews before upgrading
Aside from that, Apple has historically never optimized new ios releases for older devices until people complained in droves nor was it ever in Apple's business strategy to do so. What is more likely is that they completely ignore older device optimization as an indirect way to force people to upgrade.