Random slowdowns and battery drain

Hello,

I've got an iPhone 6s Plus that's playing up. I replaced the battery on it a while ago after the original failed (was draining too fast even when deep sleeping). I went with the battery sold by Replace Base in the UK. It seemed like they would sell decent replacement batteries.

Unfortunately, while the fast draining on idle has stopped, the phone has other problems now. It will randomly start lagging badly when trying to use it. You can barely tap anything when the lag happens. I used an app to check CPU usage and I see it's pegged at 100%. Battery starts draining rapidly as well.

It could go days without having this issue. It just comes on suddenly and when it does, it makes the phone unusable. You have to connect it to a charger to get the lag to disappear.

The battery seemed fine at 1st, then after maybe a few months this issue began.

Is this a bad battery? I've also noticed that when the phone 1st boots from a no power state, I'll see really low percentage (like 2-4%). After a few seconds, it'll jump to a higher value. It did this from the start, but I ignored it because the phone appeared to be working fine.

This looks like a case of a bad battery to me, but just making sure. If it is a bad battery, does anyone know of a seller that sells quality batteries that don't have problems? I've tried 2 replacement batteries so far and both have had issues. The 1st caused the phone to reboot all the time. The 2nd (the Replace Base one) has the issues I've described in this post.