DIAGNOSIS The 2.4GHz band is congested or a setting is throttling it. Neighbor networks and Bluetooth devices cause major interference.
FIX Change the 2.4GHz channel width. Log in, go to Advanced > Wireless > Wireless Settings. For 2.4GHz, set Channel Width to 20MHz, not 20/40MHz. Choose Channel 1, 6, or 11 manually. Apply.
IF THAT DIDN'T WORK Smart Connect might be misdirecting devices. Disable it temporarily. Go to Advanced > Wireless > Wireless Settings. Uncheck "Enable Smart Connect". Set separate SSIDs for 2.4GHz and 5GHz. Connect slow devices to 2.4GHz SSID directly.
HEADS UP 2.4GHz will always be slower than 5GHz on this router. That's normal. Its purpose is range, not speed, for things like smart plugs.