I ran into almost the same problem last year, and at first I underestimated it. Like you, I tried a plunger and a chemical cleaner, but nothing really changed, and the draining just kept getting slower. That’s when I realized it might not be a simple clog near the surface. I took apart the P-trap and cleaned it, but the issue was still there https://bens.plumbing/areas/rainier-valley/drain-cleaning/ so I used a drain snake and pushed it deeper into the pipe. Turns out there was a heavy buildup of grease and food further down. After clearing that, the water finally drained normally again. If it keeps getting worse, it’s probably deeper in the system, so I’d suggest using a snake or getting a plumber to check it before it fully blocks.