I started this thread with a lot of skepticism as to whether “high speed” might just be little more than a buzzword. However, I subsequently found this post by Polymaker:
PolySonic™ PLA
where they actually do define what they take “High Speed” to mean.
TL;DR: It means being able to support a minimum of 24mm3/s flow rate while maintaining most of the properties you would expect at more normal and slower speeds. Worthy of note: one of those properties is maintaining the same sheen level, even though the temperature is typically fluctuating all the time as the print speed is constantly changing. From an aesthetic point of view, this can be very important.
In the case of the polysonic, they tested it to flow at 29mm3/s, albeit with a volcano hotend and a Hemera extruder. Well, this is where I think their definition becomes a bit squishy, because I’d wadger they could get an even higher flow rate than that if they tested using an even more powerful hotend and extruder. Nonetheless, provided they stick to just this particular test rig for evaluating all their different filaments, then it becomes a common yardstick for judging which are good enough for the label “high speed” and which are not good enough for that designation.
In any event, it further explains how they arrived at the 300mm/s speed number. That’s the wrong figure of merit though. Rather, it’s the maximum volumetric flow rate which is fundamental and is what really matters.
If it were purely a scam, I think they’d probably label everything in their catalog as “high speed,” but they do not.