The Spectre security related updates in iOS 11.2.2 were only for Safari and WebKit (https://support.apple.com/en-us/HT208401), so it doesn't really make sense that performance would have taken a hit across the entire operating system. And similar benchmarks run by other people with a variety of iOS devices (iPhone 6+, iPhone 7, iPhone 8, iPhone X) before and after the 11.2.2 update do not report a performance drop.
It seems more likely that the device in this test was affected by the battery-related performance throttling that Apple has introduced in various iOS updates: https://www.theverge.com/2017/12/20/16800058/apple-iphone-slow-fix-battery-life-capacity
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