I have that also (speedtest on Librem 5 Birch):
Download: 18-19 Mbit/s
Upload: 25-26 Mbit/s
Just as I was about to write that the above numbers seemed fairly consistent for me now, when testing again the numbers were suddenly much worse:
Download: 3.77 Mbit/s
Upload: 5.13 Mbit/s
That was when it was locked, with the screen off. Unlocking it and starting the Usage app, then running speedtest again:
Download: 20.06 Mbit/s
Upload: 23.16 Mbit/s
This now makes me think the issue is related to power-saving features interfering with the Wi-Fi performance. That makes sense, I think, if the CPU sometimes works very slowly due to some power-saving thing, then the Librem 5 might for example be slow in sending its next TCP packet meaning the other side does not see an acknowledgment quickly enough, so the other side decides to resend data and/or decrease the TCP window size, things like that could be happening.
It would be interesting, then, to turn any attempts at power-saving to be sure the CPU runs at full speed all the time, and then run speedtest in that situation to see if the performance is then better and/or more consistent. Anyone here know how to turn off all power-saving? Maybe that question is worth an issue of its own.