I'm still on iOS 10 on my iPhone SE. Why should I upgrade? Even if there was a convincing reason to, I'm afraid performance will tank, as it often does on older iPhones getting their last update.
Security updates and significant speed improvements are sufficient reason to upgrade, I'd think. Siri Shortcuts is also a pretty incredible feature if you're a power-user.
I'm extremely skeptical of claimed speed improvements. Are there third party benchmarks? Are these improvements over iOS 11, or iOS 10? iOS 11 was an absolute horror show from that and many other aspects.