Why does my iPhone 6s sometimes not respond for 5 seconds?

Hi, several times a day when I try to wake up my iPhone, it takes around 5 seconds to respond, no matter how many times I press the home or power button. Also, when I plug it in it will sometimes take 5 seconds to respond to the charger. I haven't been able to figure out a pattern of why it happens (eg. After being asleep for a long time or having lots of apps open) but it used to be 2 or 3 seconds to wake up sometimes & now it seems to be becoming a longer delay.

I bought the phone from my uncle who hadn't had it long but had had the screen replaced (not by apple). I've taken it to a great phone repairer I've used a lot before but they have never heard of the problem & don't know what's caused it. I tried enquiring at an apple genius shop but they blamed the screen being faulty because it wasn't repaired by apple. I tried to explain that it's not just the screen because the delay sometimes applies to charging as well but they refused to help.

Any answers would be greatly appreciated. I'm very reluctant to buy a new phone because this one works perfectly if and when it turns on but I'm at my wits end with the unpredictable delay.

Many thanks.