You were right conspiracy theorist... your old phone does get slower and starts acting wonky when a new phone is coming out.
On Wednesday, Apple admitted that a software feature it released last year makes your phone operate more slowly. But the slowdown is designed to help offset problems with the aging lithium ion battery in your device.
As batteries get older they don't hold their charges as well as newer batteries, so Apple is intentionally slowing down the processing speeds of older iPhones in order to prolong battery life.
Apple's iOS software--starting with last year's iOS 10.2.1--included better power management capabilities.
In a statement Apple said, "Our goal is to deliver the best experience for customers, which includes overall performance and prolonging the life of their devices. Lithium-ion batteries become less capable of supplying peak current demands when in cold conditions, have a low battery charge or as they age over time, which can result in the device unexpectedly shutting down to protect its electronic components."
"Last year we released a feature for iPhone 6, iPhone 6s and iPhone SE to smooth out the instantaneous peaks only when needed to prevent the device from unexpectedly shutting down during these conditions. We've now extended that feature to iPhone 7 with iOS 11.2, and plan to add support for other products in the future."