Morning!
You should be able to see speeds north of 2.5mph easily enough, especially in the conditions you described. That motor is meant to yank around even the biggest of walleye rigs.
I would disconnect and load test each battery individually. While it may show full voltage, the reserve may not be there to handle the high current draw. If this is a new problem this season, I would point pretty confidently to this.
Another possibility if this has been an ongoing problem is that you have a large enough TM plug at the bow. If the boat was originally wired for a 24v system, that plug may not be handling the current load either. I have seen where conversions to 36v, someone hits the rabbit button on their remote and all of a sudden the TM plug starts burning up from the sudden current load. You should have a 3 prong, twist lock type plug at the bow or a screw in battery tender brand plug, not a round 2 pin type connector.
Good luck!