Some people keep low power mode on their iPhone all the time. That’s an awful idea. You can see why above. On the right, you have my iPhone X running Geekbench’s CPU test. On the left is the same iPhone X, but this time with low power mode turned on. You can see my iPhone X became slower than even an iPhone 7. If you’re keeping low power mode on, it’ll have the performance of an older iPhone.
Low power mode slows your processor down so it doesn’t use up as much power. With low power mode on, your phone is about 56% as fast as it usually is for multithreaded tasks (most computing tasks), and 64% as fast for single threaded operations. You likely paid a lot for your phone, keeping low power mode on is only going to slow it down. However, this does not apply to the super low power mode for the iPhone X I mentioned before, which remains the best way to save power on the iPhone X if you don’t want to hurt performance.
This is why low power mode should only be used in emergencies, as Apple suggests.