Yes the venturi even when clean severely restricts the flow. Clogged and it gets worse. They need to be removed and cleaned periodically as well as the valve replaced if it is leaking. Some models have an adjustable bypass to allow more GPM through but the more that bypasses, less air gets added to the stream. What I do is periodically open up the bypass during a manual backwash.
I have considered going with an air-pump system but they are expensive here.
I also thought of going with an Excalibur system but there was not enough technical info on there site to satisfy my curiosity of how well they would work.