Rocker, width and fin pod. Most 5' wakesurfers are in the 20"+ width range...that grom is what 17? Tail width in particular is no doubt much smaller.
Also the grom board no doubt has a fairly substantial rocker to allow for the drop in. A typical wakesurfer will have under an inch on tail rocker and less than 3" of nose rocker. My guess is your grommet board has more on both the nose and tail rocker. That will tend to slow the board down as it attempts to push water out of the way.
The last thing that I've seen on most grom boards is the fin positioning...a bit more toe-in than you'll find on a typical wakesurfer - helps with turning, but induces more drag.
In the ocean on a shortboard folks don't just stand and cruise, unless the wave has a lot of juice, shortboards are ridden - lots of turning and pumping which creates drive. When folks are learning to wakesurf they are mostly standing there.
I know it's easy to look at something like a Shred Stixx and think - same as a grom board, but lots of time and energy have gone into most of the more popular independent manufacturers offerings. They also know how folks ride and manufacture to that audience.
Hope that helps.