iPhone 8 Plus has very slow wifi speed

My iPhone 8 Plus, running iOS 12.0.1, has slow wifi speed. I tried checking the speed using another iPhone 8 Plus connected to the same network and same server, using Ookla. It picks up almost twice the speed of my 8 plus. I tried checking with different wifi networks also. The result is the same.

I’ve tried factory resetting and restoring the data. I even did the apple diagnostic test and it showed no problem with the wifi.

Do you think it’s an antenna issue? Or could it be a software problem? I’ve been facing this issue since iOS 11.