Depending on the installation and type of feed regulator, when drum level was high feed was reduced, when low, feed flow was increased. All that happened downstream of the pump. When the flow was reduced, pressure increased and pump speed decreased because it is a positive displacement pump. Look up boiler feed regulators.
Many of the early systems that used those pumps for feed were also fitted with injectors that used steam to accelerate feed water and increase its pressure enough to overcome drum pressure. Look up steam injector as well.
The water tender has the ability to increase or decrease feed rate and help maintain drum levels, he worked in coordination with the fireman who managed the number of burners in response to load.
A working steam plant is a symphony. Every component works in harmony with the others. nothing happens in isolation (except for a sudden failure of some piece of machinery) and nothing happens instantly. Changing one parameter may have an effect that does not show itself for many minutes later.
An old time water tender knew what was going on and was capable of playing his instrument.