Advice needed, HELP 2008 sun tracker rengency 22 90 hp merc sluggish

[putt putt boom]

Hey everyone, first post here but have been a fan for quite some time. I have a 2008 Sun Tracker Recency party barge 22 with mercury 90 powering this bad boy. My problem is this: at WOT (4900 RPMs) I'm only doing 16 mph via GPS on my phone. The engine sounds like it's running fine, recently tuned, prop is 13.75X15. 2 inch logs with lifting strakes. Everything I read says I should be at least mid 20's. PLEASE HELP!!!!