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If your static pressure is around 40 psig, that is ample. What you are seeing, though, is somewhere in your system there is a big flow restriction. Without going over the plumbing rather carefully, it's hard to say what — but it could be as simple as a partly closed valve somewhere.

Before you go the expense and trouble of putting in a booster, you need to find that restriction — because if the booster is on the low pressure side of the restriction, it will work poorly if at all. They are designed to take ample water, but at low pressure, and boost that — but you don't have ample water.