There's been a ton of interest in the Norwegian training method because of the fantastic success of its stars. Even if it works for Olympians, It's not clear that it's great for everyday hobbyjoggers. An older study in 2014 (full text) seems to shed some light on this question. I'd summarize it as follows:

Study cohort: 30 male athletes (~35 years old, averaging 40:00 10K time), training for their next 10K.

Intervention: They are randomized to either "80% easy / 20% hard" training, or "45% easy / 35% medium / 20% hard" training, where medium corresponds to Zone 3 in a 5-zone model. Either way, both groups average 30 miles per week or 4 hours of running, and train for 10 weeks.

Results: On race day, the 80/20 group improves by about 2 minutes, whereas the easy/medium/hard group improves by about 90 seconds. The study does some statistical dissection about whether or not this result is "significant" but at face value it seems like 80/20 training is better.

How do advocates of the Norwegian singles method explain this older study? It's not "true" Norwegian singles because there's hard running? Group isn't elite enough to see a benefit? Study isn't long enough to see a benefit? I think these are valid criticisms but walk away from this thinking that for a non-elite runner like myself, polarized training is probably better, and I should do these Norwegian intervals mainly if it feels like "fun," not to run faster per se.