SD = Standard Definition
If your on a Windows PC check out this tool to measure “Disk to Disk” performance => Totusoft | LAN Speed Test
The results will probably be limited by your wifi speed.
As you have set the WAN port to a LAN port, is this now a stand alone network, or are you connecting wirelessly to another router? IE Is this unit a wireless bridge of some type? If so, it’s probably worth elaborating.
Just because you set your router to run at 300M does not mean the client (Laptop) is. You may need to check the config of the WLAN in your laptop. You may need a wide channel set on both devices. Your wireless is basically capped at the speed of your slowest connected device.