Drano Kitchen Max Gel Drain Cleaner 770mL

I've used this product in several occasions as my kitchen sink has old pipes and a slow drain and, when I run the water continuously, the sink cannot cope and the water pools and bubbles up. I've  tried other products in the past and I barely got any result. I've used this and the slow drainage stops. Half of the bottle is what I need and what the package recommends to use. However, unlike the instructions on the package, I leave the product in for at least an hour, not the 15-30 minutes recommended as I did this in the past and didn't work as well. I got the bottle today, I used upon arrival, and my sink drains normally.