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First, make sure that you’ve selected 802.11n for the operating mode and WPA2+CCMP for the encryption. If you are running the known-insecure and obsolete WEP and TKIP, the rates are limited to the legacy 54 Mbps modulation rates (not to mention it being trivial to crack your network without knowing your pass phrase). Unchecking the “Legacy rates” box is helpful, as anything that still requires those long-abandoned modulation rates will drag down your entire network (802.11n was ratified in 2009, and has been around on devices for probably 15 years now).

Second thing is that the difference between “theoretical” bandwidth and real throughput is quite a bit.

To start with, 40 MHz channels are unusable pretty much anywhere than you can see another house. So you’re immediately down to 72 Mbps from 150 Mbps per stream, modulation rate. Now, many clients don’t support multi-stream, so they’re not going to have any chance to double that, even though the AP is capable of it. Next, you can only get ~80% of the modulation rate as data throughput in laboratory conditions. So you’re down to ~50 Mbps for 802.11n. Add in that if you’ve got other clients, or other APs in the area, those time slices are used by someone else, further dropping your client’s throughput.

So 30-40 Mbps is pretty typical for 2.4 GHz operation.