Your 16x16 .cap actually uses a 16x18 texture That's not power of two, so power of two rendering is disabled! If you resize the texture to actually be 16x16, the framerate goes up.
Just to clarify some of the comments here, in practice the framerate for tiling power-of-two textures (16x16, 128x128...) should be roughly the same. Power-of-two textures can be drawn more efficiently - there's basically a single command which is "tile texture N times". That doesn't work for non-power-of-two textures though, so instead for each and every tile Construct has to issue a separate "draw this texture once here" command. That introduces a very large overhead of command data going to the GPU which is the main cause of slowdown when many tiles are visible on-screen.