All I can relate is that I favor the Helix over the Brompton and the Bike Friday. Have also had A-Bike (a mistake), Strida, Bickerton, and a Dahon previously in my stable. Those have all gone through the years. My experience with the Helix is such that were I faced with the initial purchase choice, I would not look at any folders with a sideways hinge or anything smaller than a 24" wheel. I hope that's not a recency bias on my part; I don't think so as I spent just as much on the other bikes as on the Helix. These days the Bike Friday has been sold. OneBrompton has been given to my wife, and another given to a family member. So the Helix is the only folder left. It's a better ride for my ~10 mile commute which was do-able but a bit of a slog on either the Bike Friday or Brompton.
re: efficiency (previously mentioned as speed)
If I rolled up to my Saturday morning group ride on my 24" Helix and told the guys that I was going to beat them on their 700c road bikes, they'd look at my cassette, my single 42 front chainring, and my tiny wheel and laugh; then they'd ride me off their wheel even more than they usually do. All things being equal, in the world of upright bikes, I am not aware of "tires construction, width, pressure (that's some magic tires to erase a 7mph deficit), frame efficiency, position on the bike" that I can apply to the 24" Helix that will surmount the speed deficiency compared to the 700c bike. I could hang with or beat them with the small wheel were I on a fairing recumbent but then it's apples and oranges.
The one thing I do find unfortunate is that Helix hasn't been able to scale up quickly. But completely understandable given current headwinds. I'm guessing no one laments that more than Helix's founder. Still, they're on pace, based on the last update, to complete all deliveries by end of July. Hoping Q3 rolls out new products etc. There's lots of good favorable feedback on FB for Helix - Hong Kong Helix Club especially has some cool mods. Alternative wheelsets (at least one carbon set that I saw), handlebars, racks, etc. Lots of pics showing off-road use with occasional bikecamping. It doesn't seem to be holding folks back from enjoying the bike on any terrain. And it is actually encouraging a bit of creativity in personalizing the bike. Which is something that can be said for the Bromptons and Bike Fridays of the world too. So pick your poison and drink hearty.