First the disclaimer that the OP's butt dyno might need re-calibrating, just because a tire feels slower doesn't necessarily mean it is.
At the same time, there might be something to it, as the SK+ is ~40g per tire heavier than the "plain" SK, which isn't a dealbreaker, but all of that weight is the additional puncture resistant belting. I only have experience with the SK, which I didn't notice to be particularly draggy, at least for what it is. I have ridden both the Panaracer Pasela and the Pasela Pro-Tite, and the Pro-Tite (with that extra protection belt) did feel... clunkier.
So in the realm of internet guessing, the SK+ is probably a bit slower than what the OP is used to, but probably not as slow as he's guessing.
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