Having gone through that tunnel a couple of weeks ago, my experience was a bit similar to the OP's (not being followed & harassed by pratt in a hurry, but losing speed). After a while I thought of dropping into neutral & / or giving a burst of reverse, which seemed to improve things for a few minutes each time. So I think that leaves, no doubt being drawn into the tunnel by boats going through it, probably are at least partially a cause of the problem.