I will pretty much repeat what the others have said. On the rear cluster, you are not going to get anything smaller than an 11 tooth cog. There is no way to get a higher gear that way.

On the chain rings, you can probably swap out the 48 big ring for a 50 or 52 big ring. But even that is only an 8% increase.

The probelm you are probably having is that you do not know how to "spin". Going fast in cycling requires being able to maintain high RPMs. Some people like pushing big gears at relatively low RPMS. I think that Gregg Lemond ran RPMs in the high 80s. Other cyclists spin and often run a bit over 100 RPM. In a sprint, one can hit over 150 rpm.

Your top gear is 118 chain inches. That comes out to a speed of 35 mph at a 100 rpm. It is still over 27 mph at 90 rpm. I have a feeling that your RPM is probably at 80 rpm or even lower.

You could get a bike computer with an RPM output and work on your spin. Being able to maintain higher RPMs is pretty much the only way you are going to increase your top end.