Did you mean the Flint 1 or 2?
Does the GL Flint repeater to the ONT? Or the Flint connects to the ONT via the wire cable?
What firmware version does this happen? Please try to the 4.6.4-op24 firmware to see if works better from GL.iNet download center
Having the same issue.
This is with my normal provider
600mb
and this is with Flint 2
It used to worked great but now I am getting 10% of the speed
Which firmware are you using on your Flint 2?
How did you measure the speed? Wi-Fi only? What happens if you check by ethernet?
I am using the latest firmware 4.6.4 and both measurements were on wifi from the same spot. The modems are in the same spot as well. I will try to measure them via ethernet
Also may try the v4.6.4-op24 firmware, probably that open-source wireless driver better compatibility
Same issue here. I have 300/300 with AT&T fiber. Getting 300 down and only 150 up with Flint 2 across different devices, while it's 300/300 with the AT&T router.
Hmm i see this post move to all kind of angles.
a word about mtu:
If the mtu is default usually there should not be a issue when 1500 (or DSAs default), except when a vpn is involved often an mtu of 1384 is recommend for a client, but i have seen situations you may want to crank this value down to 1280 mtu especially when wifi was involved.
1240 mtu is the minimum requirement for ipv6.
In my server setup i have wireguard set to 1550 and wan 1580, at home i run a vxlan tunnel over wireguard so it must have a high hierachy of mtu, but on my road warrior the mt3000 i take on vacation with often expected bad wireless throughput i had to set the mtu to 1280.
One of the major issues here is that it immediately fixed the upload being higher than download effect, this could be a indicator for you to if mtu was a issue 
So what i want to know: do you use any protocols like wireguard, pppoe or very complex setups like ethernet vpn?
A word about link speed:
The wifi chip (mt7986) offloads packets to the cpu, the chip can do that very effective, but the cpu is also bound to the DSA switch (Distributed switch architecture).
With other words the chip can do more, but it is bottlenecked to what the cpu can handle, so the maximum what wan can archive is what you likely can expect as real link speed on wifi but reduced due to certain noise factors in wifi itself.
As for your issue that it drops lower:
Are there external noise factors such as a connected usb stick?, a fan?
Have you tried setting the country via luci, 80mhz width, and the lowest supported channel and then increment it with the next lowest possible option?
There is also a option in advanced tab also in luci -> network -> wireless -> ap? (Click edit), called Disassociate On Low Acknowledgement try to uncheck it.
On the devices with the issues, do they use features like power saving mode?, you can try to disable it.
Have you tried without bluetooth active on these devices?
Could the weaker WiFi performance be related to the WiFi power being lower on Flint 2? I also have Slate AX and it shows a higher signal strength and transmit power at 30 dbm vs only 20 on Flint 2. Both with USA selected
It's strange the difference between the two.
On my own Flint 2, i have country set to netherlands with 23 transmit power, but i can have a throughput around 800 mbs/s even though link speed reports around 2.4 gb/s, though i use a self compiled snapshot from openwrt master maybe that can give the difference?, it's worth looking into the op24 one.
However i believe transmit power also gets limited when dfs is active.
So even though transmit power can play a role, when you are close to it, you still should see some speed.
I only noticed issues on more earlier versions there was a regression and maybe still is when the device connects from far away and then comes close the speed degrades until 50mb/s download and 50mb/s up, reconnecting the device on wifi fixed it for me.
But the issue is rare to replicate.
