Although I got the impression of the Silhouette being quite fast, I did not look in detail yet to the speed-aspect of the kayak.
Last week, being in the mood for a workout, I took her along on Wednesday-evening for a trip to the club planning to paddle 18 km's.
Being fresh, I started my warming up and measured 9- 9,5km/h; being easy. I tried also to paddle at 7- and 8 km/h but that was difficult to maintain, as this was too slow for me at that moment.
Being "on steam" after 4 km's, I started varying my speed:
- Paddling 8,5 km/h felt as a very natural speed in this kayak: being a speed that I could just maintain forever.
- A good exercise-speed was 9,5-10km/h: Creating sweat on my head. But I must say that the 10km/h is not something I could go on with for longer periods.
- Going for a full sprint I reached 11,8km/h with the first attempt. But this was something I could not reach again with the second and third attempt; it stopped than at 10,5km/h.
- Paddling on the lake against a 3Bft-wind I reached, as exercise pace, an 8,5/km/h, sometimes increasing to 9km/h.
On my way back my energy level was going down (beginning of the season 
) and after 15km I was just tired and had to work for it. I found it interesting that I ended up at the 8-8,5km/h, which felt so natural for this kayak. I could speed-up to 9km/h though: but only by using more trunk-rotation. However this rotation did nog feel very comfortable anymore because my back started being a bit irritated. I did improve the backsupport-block in the seat quit a lot already, but the customising was apparently not yet enough for my back.
As I paddled the Silhouette with improved knee-contact on flat water tonight, I noticed the benefits as I had much more control, better manoeuvrable and better stability while edging.
I should have done that earlier.
Summarising:
In flatwater-conditions and light winds a good cruising speed is 8-8,5 km/h
But also 9,5 km/h can be maintained for some time while paddling at exercise-pace.
Going above 10km/h felt if going above the maximum speed of the hull-shape, Clearly this can not be maintained for longer periods.
With these figures I think you may conclude that the Silhouette is not a slow kayak.
I would like to comment that the speeds as mentioned, depends of course on your fitness-level and technique, but I think they apply to an average, trained paddler.