Depends on what constraints you set. If you decide on the # of vehicles per hour, and can correctly predict the speed, you can calculate the total number of trains needed. Lower speed will results in a larger required number of trains and higher operating expenses. But if you are willing to pay that price, you still can provide the desired frequency and capacity.

If your total number of trains is fixed and you can't get more, then your statement is correct; higher speed results in a higher frequency and capacity, because each train makes more trips per day.