We always knew that patching the recently-discovered Meltdown and Spectre vulnerabilities will involve a drop in performance, especially in older devices. The magnitude of the drop is still being examined, but the early results don’t look good.
In at least one case, an iPhone 6 saw its Apple A8 lose 40% of its CPU performance after going from iOS 11.1.2 to 11.2.2. The test was run by Dutch entrepreneur Melvin Mughal who got the following results on his phone before and after the update.
Before Spectre update
After Spectre update