Speed control of 110vac AMT Pump

The most economical way is to install an appropriately-sized globe valve in the delivery pipe, and wind the handle until the flowrate is that required. A 1in globe valve ought to be available for less than GB£10.

Although you do not answer the OP's specific question, I'll bet he slaps his forehead and says, "Why didn't I think of that" when he reads your response.

GA.

He might want to monitor heat build up in the pump, when throttled back, but it probably won't be a problem.

That is certainly the lowest initial cost, but it might be worth looking into the potential running cost savings using a variable speed drive.

A "quick and dirty" calculation of the Hp in 10 GPM at 100 psi (what pressure are you running at), is about 2/3 Hp, or about 500 watts. How many hours do you expect to run at the reduced capacity? Each 2 hours of running at 20 GPM (30 -10) saves 1 kwhr, at least $0.10, but probably more.

Which is why a valve is more economic.

How many hours a year will the pump run at 20 GPM (instead of 30 GPM)?

Assuming that a GBP is about twice the value of a US dollar (and electric rates are in the same ballpark), and a variable speed drive for something like this is in the range of $100 (I have no idea of current prices), 4000 hours of operation at 20 GPM will pay for the variable speed drive.