Hello. I am hoping I can help/get help here. Understanding that there is no "up shift", I am experiencing the same problem. Even when engine is warm:
From stopped, giving steady increase to throttle, RPMs rise to 7K very quickly, but speed is very slow and doesn't increase much at all for 30-45 seconds. Then, at 10-15 MPH and 7K RPMs, it feels like an up shift happens; RPMs drop to 2-3K and speed begins to increase fast. Normal engine sound and speeds then as long as you keep the speed up over 20 MPH.
Checked intake, impeller, etc. All seem clear. Fuel lines don't appear blocked or kinked.