If you are asking about an Airport Express......this product has only 100 Mbps Ethernet ports, so the maximum theoretical speed that it will ever deliver would be 100 Mbps. The Express normally delivers about 60-70 Mbps on a 100 Mbps connection on a good day.
A higher speed Internet connection will not help the AirPort Express.
If you are asking about an AirPort Extreme......then there is a problem somewhere that will need more troubleshooting......since the Extreme can easily handle a 300 Mbps Internet connection and deliver about the same speed to connected devices.
For example, I just ran a few quick checks on the AirPort Extreme here with a 300 Mbps Internet connection. The AirPort checked out at 315-320, meaning that the provider is delivering a bit more than they promised.