iOS 11.4 slowing down iPhone

Seriously?

I have an 8 and I’m having exactly the same problems

Try a hard reboot, that often fixes issues after an update. It could just be an app that isn't playing well with the new iOS.

Look under battery in settings to see what is draining the battery. With my wife's iPhone, it was the native camera app, even though it wasn't open or in use, so couldn't be closed (hope that makes sense). The hard reboot fixed it.

This is hardly helpful or constructive. The reference to worse is in a particular context, that being the speed of the OS on particular models. It was not suggested that the new OS is worse in its entirety and that the benefits do not outweigh the cons. It is not fair on that basis to accuse people of lacking logic, perhaps their language could be more precise. What is illogical is to state that any "problems that someone has can be resolved in one way or another and if it is a software issue it is FREE" without explanation of what you mean or how they may resolve them.

No doubt you mean well, but its better to be critical and constructive, than just plainly critical.

I am saying that the original poster should update to iOS 11 and that we can help resolve any issues that may arise. If the battery is bad it was bad before the upgrade. There are many steps that can be taken to resolve most software issues. Posting things that are disparaging and the opposite of what actually needs to happen is what is disrespectful. I could care less either way but to blatantly give false information based on someones unfortunate interaction with a device is not what this technical forum is for. Everyone should always update their software because of security features. The software they are holding so dear to had countless updates too. Most times it is not because it is better but because it is familiar.

iPhone 6 has slowed after iOS 11.4.1 install. Really annoying. WiFi seems slow and apps hang or don’t open. Come on Apple. Sort it out.