In your email, you mentioned eliminating your unions. If I were you, I might replace them with unions that had a more positive connection, but they are not your flow restriction.
I see two likely culprits.
The Hayward chlorinator will restrict ALL flow -- if you remove it, look though it. The hole that ALL your pool water has to flow through is about 1" in diameter. That's the likely cause of your 52# filter pressure. I used to throw those things away, any time I found them on a pool with a pump larger than 1/2HP.
I'm going to be take some pics at my wholesalers. I'll see if I can get a picture of the interior of that mis-designed piece of pool gear.
Second, if the green corroded ball valve is on your vacuum line, we may have found the guilty party. Such valves often have a small port. If that valve is on your skimmer line, it will mean that water always finds it easier to flow through the main drain than the skimmer. In that case, you'd have very poor suction pressure. (Are you using a pressure side, or a suction side cleaner?)
Ben
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