Downloaded the GCode. Read it into S3D and stepping through the graphical preview single line by single line while viewing the speed, I may see what's causing your problem. The CW and CCW outlines are a single segment/side drawn at ~18mm/s with a few short movements within the turnaround corner at ~104mm/s, This is followed by the fill pass which is a straight line (not jaggies) consisting of ~32 (rather than a single) segments/side executed at ~104mm/s, followed by a movement to the adjacent corner at 233mm/s, some fill in that corner at 39mm/s, another short movement within that corner at 233mm/s. At this point the process repeats.
Unfortunately I've no idea what massaging/interpretation S3D may have done to your GCode wrt speed, or anything else. That said, the output I was previewing did NOT look like anything S3D has produced to date, so maybe it is representative of the actual GCode content. I have not tried printing your file.
If S3D's preview function might be helpful to you in understanding this problem, I believe they have a 14 day refund period.