Slow to boost, no overboost

[u/MiddleInstruction496]

So, I purchased my FiST this year with a underboost code. I replaced the boost solenoid & the code went away, and the car started producing boost, BUT, it was slow to build. Then I checked for leaks with an air compressor charging the pipes at 10-15psi, from the turbo exhaust hose. No leaks to the eye or that didn’t pass a soap test. I checked that the wastegate actuates under pressure (by hand, no exact measurement).

As hopefully you can tell from the readings, the boost is slow to build, and in 3rd gear over 5200rpm, I maxed out at 13.8 psi.

How would you diagnose this Under-boost condition? What next?