IFixit Replacement Battery Not Holding Charge

About 2 or 3 months ago I ordered a replacement battery from IFixit for my iPhone 6s Plus. It has worked fine until recently all of a sudden. My problem is best described as once the battery level drops to around 60%-30% (it varies), my phone slows down to a crawl and is practically unusable despite the battery percent stating the battery isn’t about to die. There’s a 1 in 5 chance that if I restart my phone the percentage drops to around 10%, as if the percent meter isn’t calibrated. It was on 1% last night and it was fully charged in the morning, only to drop from 80% to 10% within a couple hours of use. Any suggestions on what I can try or what to do would be appreciated.