iPhone 6 Performance

So, I purchased an iPhone 6 last week (second hand) which was fairly cheap as I simply wanted a bigger phone. Anyway, it came running iOS 11.1 (since updated to iOS 11.2.1) and I assumed that iOS 11 was just really slow on the phone, however after a bit of digging online I came across speed tests on YouTube showing the iPhone 6 running very close to iOS 10 speeds HOWEVER mine is very, very slow compared to the ones in the videos.

So I dug around online even further and found that there is an app that tells users the clock speed of the chip inside, and as far as I'm aware the iPhone 6 is 1.4GHz... But the clock speed app shows mine running at 600Mhz! I didn't want to trust one app so I installed Geekbench and noticed the single core score is just above 800 and the Multi-Core is just about 1200. This is significantly lower than other iPhone 6's I have seen.

On many forums, people say replacing the battery helps as Apple supposedly throttles the CPU when the battery has a certain amount of wear, however, my phone came with a brand new, replaced battery (0% battery wear according to a battery health app). So I'm not sure what is going on and wondered if anyone could help?