The 2.4ghz band is overloaded. You are probably getting interference from somewhere (baby monitors, virtual networks, neighbors, Bluetooth, and nearly every wireless device ever).
40mhz wide channels frequently default down to 20mhz channels. With a 2x2 client, PHY speed is 144mbps (you’ll never get this).... add in interference + overhead... anywhere between 60-100 is a good result.