Try comparing the internet connection speed with your Samsung phone and another mobile phone on the same network. Don't try comparing to a laptop or any other device. If at all possible, try with another Samsung handset.
I upgraded my Samsung S9 plus to Android Pie with no negative effects on my Wifi speeds like you explain.
I noticed that the wi-fi on my phone is 760 KBps when I my laptop is connected to the same connection, even when my laptop is idle. Wi-Fi on my laptop tests above 80 MBps. If I turn off my laptop, the wi-fi speed on my phone jumps to 40 MBps! I am thinking of buying a new router soon. Is there anything I should check before spending money on a new router? BTW, I have turned off bluetooth with no effect. I have a tablet which runs well on my Wi-Fi connection when my laptop is connected. When I use data on my phone instead of connecting to Wi-Fi, it is quite fast!