At the same pressure and with a similar structure, wider tires have a lower rolling resistance and are faster.

This is the reason why road race bikes use wider and wider tires, moving from (long time ago) 19mm wide tires to 21, 23, 25 and nowadays 28 or 30 or even 32mm wide tires. The reason why they were previously using narrow tires was aerodynamics but bike manufacturers discovered that aerodynamics was not mainly defined by width but more by tire+rim shape and that it was possible to have as efficient wheels with wide tires and well shaped rim than with narrow tires and therefore having both a low rolling resistance an an excellent aerodynamics with wide tires and rims designed for wide tires.

So, due to the very limited availability of 24" tires up to 40mm wide, a 20" wheels with a wider high performance tire will be faster than a 24" wheel with the few available narrow tires.

Note also that the use of wider tire on a 20" wheel reduces even further the overall wheel diameter difference, for instance Helix with max 40x507 vs. Brompton G-line or Bike Friday with max 55x406: the overall wheel diameter difference drop to only about 12%!