I took a ride yesterday and the MAX was 34.7 mph, on a downhill, peddling robustly as I approached the associated uphill section.
That was with 56/11t gearing, 2.00 Big Apple tires, on asphalt roadway pavement.

The "safe" max speed on folder is probably pretty darned subjective. What bike, what gearing, rider experience/fitness, what tires, what surface condition....etc. etc.

It's not just what gravity and peddling will allow, but also wind resistance.