I haven't paddled the "Guillemot", but I have spent quite a bit of time in Nick's "Night Heron", which also tends to slap into choppy waves.

It probably does slow it down a little bit in certain conditions, but I don't know whether it would be enough to get excited about. I found it a minor irritation in some conditions, but I certainly never thought of it as a big problem and never felt it was slowing me down enough to matter.

There are always trade-offs in kayak designs and that is certainly true of that tendency to slap a bit in choppy water.

It happens because the bottom part of the hull gains width a bit more quickly than a kayak with a very fine bow.

Some of the benefits of that shape are that the kayak has significantly greater stability than you would expect for a 20" wide kayak and it has less of a tendency to bury the nose when running down the face of the swell.

There may be other benefits in the way the kayak handles too.

In some conditions the Night Heron and I expect the Guillemot, would have the advantage over other kayaks with a finer bow that didn't slap as much in choppy waves.

You can't have everything in a kayak design. Usually a benefit in one area means a disadvantage in another.