I'm with Hank - start with venting. I did the one-way valve mode on the filler caps, and my wings consistently drain much more quickly than factory original. Also, I had the exact symptom you are talking about at Harris Hill last year with a pinched vent tube in the right wing. Also, once pinched, they are more likely to get pinched again since they develop a memory crease.

As a quick test with the wings full, put tape over the vent outlet in the wingtip while you open the valves, then peel the tape. There should be a very distinct improvement in dump rate.

P3