When doing a speed test between Houdini and Houdini Engine, Houdini was a lot faster. Simply plugging a grid (500x500 rows columns) into a mountain sop and animating the offset. Houdini was using 20 - 25% of processing power whereas Houdini Engine in Maya was using 5 - 9%.

The FAQs say speed difference is negligible. Am I missing something?

Houdini 14
Engine 1.9
Windows 7 64

In that setup, one cause of that might be the extra attributes that are sent in as input. When the Maya plugin convert Maya geometry into Houdini Engine, it adds some extra metadata to the attribute. In this case, it might be the “maya_shading_group” attribute, which is used for material assignment. If you're not interested in using material assignment, you can try deleting that attribute. Try out the attached asset.

Ah but I tried without using a Maya input, the grid was already inside the asset so that can't be it.

If it's only output, then it's likely that most of the time is spent in the Maya side, either the Maya plug-in or Maya itself. I did a test just now by comparing two things:
1. a 500x500 Maya plane, and made it frame-dependent so that it'll update every frame
2. a Houdini asset with a 500x500 plane, and made it frame dependent as well
In the first case, I got around 3.2fps. In the second case, I got around 2.4fps.
Given the overhead that the Maya plugin adds, I'd say the two are reasonably close. The lower CPU usage is from the plugin passing data into Maya.

The FAQ was probably oversimplifying things. The overhead from Houdini Engine layer itself may be negligible, but the time spent getting the data from Houdini Engine into the host (i.e. Maya) is probably host-dependent and a bit more significant.