RPi3B+ very slow WiFi

[Bosse_B]

I have just bought a new RPi (my 10th or so...) and this time it is an RPi3B+
When connected by Ethernet cable I get very good network speeds as tested with the Swedish "Bredbandskollen":
Download: 224,220 Mbit/s
Upload: 128,090 Mbit/s

But when I switch to WiFi (pulling the Ethernet cable out) I get very disappointing speed:
Download: 4,417 Mbit/s
Upload: 29,752 Mbit/s

I thought that the WiFi on the 3B+ would have been improved but it seems like it is not at all.
Where is the WiFi antenna located on the board?

[HawaiianPi]

The antenna on the 3B+ is incorporated into the PCB itself. It is similar to the design used on the Pi0W, which proved superior to the chip antenna used on the 3B. In the picture below it's on the left edge of the board next to the WiFi module shield.

3BPlusPCBantenna.png (79.37 KiB) Viewed 310 times

In testing the 3B+ antenna outperformed the 3B and Pi0W antenna (so your problem is something else).

[Bosse_B]

I am running OSMC on this new box so setting up WiFi etc is a bit different...
Anyway I pulled the wired Ethernet and then I used the MyOSMC applet to change WiFi to use the 5G network.
Now the results are as follows:
Download: 100,106 Mbit/s
Upload: 83,511 Mbit/s
As can be seen a LOT better.

So I switched back to the WiFi 2.4 G network again:
Download: 37,604 Mbit/s
Upload: 16,488 Mbit/s

Clearly the 5G WiFi is what should be used here.

When I did the previous tests I had my RPi tie-wrapped to the case of the media hard drive (a 2.5" 500 GB USB drive) and the case is metal. So I have now slid the RPi off of the case a bit to expose the antenna end to free air...
It seems to improve the network speed somewhat, here is when I have slid it back on top of the drive case:

Using the 2.4G network:
Download: 24,017 Mbit/s
Upload: 15,567 Mbit/s

Using the 5G network:
Download: 106,336 Mbit/s
Upload: 79,034 Mbit/s

Seems like the metal case affects mostly the 2.4G band...