I had a Samsung s8plus and would always get a speed of between 300 to 350 no problem at all.
Moved over to the new S9plus and I get between 125 to the best being 180 and that's on the 5G network even on 4G the same.
PLEASE SAMSUNG TAKE NOTE AND GIVE US A ANSWER OR A UPDATE FIX PLEASE
Indeed, after turning off the bluetooth i get the desired (max) speed on my home wifi. With my wife's Note 4 i always get the max wifi speed. It is UNACCEPTABLE to get low wifi speed with the flagship of Samsung (S9 plus) unless i turn off the bluetooth. I really need the bluetooth always on because i use it a lot. Samsung you must take action to solve this problem.
I have had this issues for 2 months. Replaced modem, took phone into Geek squad, nothing. Crazy thing, when geek squad connected it to their wifi, phone was super fast. As soon as I left, some thing. I have done all the recommendations 4 times now and no change. Even my data is super slow.
I have Wifi mesh network in my home and 100mbit/s internet connection. My wifi works on MacOS and PC perfect, but in my Samsung s9+ there is massive speed problems!! At first I got about 60 mbit/s but speed slows down sometimes below 5 mbit/s. When I test it from Mac OS speed is continuesly about 90 mbit/s. This is really bad problem! because I have to use wifi connection that I can connect other devides in my network. This is my last phone from Samsung if you cant do nothing to this issue!!
I had the same issue and here is what I did to fix it:
Hold down on the WiFi network and select network settings, then advanced options then IP settings and change it to static