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 dont 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.
Both single and multi-core performance are strongly affected and memory access took a hit as well. That was to be expected as both vulnerabilities exploit the CPUs cache.
Since the test was run on the same phone, the scores are not affected by the battery-related slowdown. That does make it harder to collect more data, however.
For example, heres one result from Geekbenchs database where the scores remain on pre-update levels and another one, which is closer to Melvins numbers. There are more 11.2.2 scores like that, but its hard to tell how much of the inconsistency is due to throttling due to an old battery.