The place to start checking things would be your modem. Temporarily, disconnect the AirPort Extreme and connect your Mac directly to the modem using an Ethernet cable and run a few speed checks that way.
What results do you get? Whatever that might be, the AirPort Extreme cannot deliver any higher speeds than it receives from the modem.
Once you have run the temporary tests, you can reconnect the AirPort Extreme to your modem.
It would help if we knew the make and model number of your modem, since different types of modems tend to behave differently with the AirPort Extreme. Depending on the type of modem that you have, speeds might be limited to 300-350 Mbps or speeds might be up to 700-800 Mbps.
We are talking about Ethernet connection speeds here. Wi-Fi speeds will vary wildly depending on a number of factors which we can discuss later, but it would not be unusual to see Ethernet connection speeds at say 500 Mbps and Wi-Fi 100-150 Mbps.