My commute is 30 miles round trip. When I get to work in the morning, my clothes are completely drenched. Soaked in sweat. It's that way generally from May through September around here. So I wear synthetic cycling gear. It gets wet, but I rinse it out when I get to work, hang it up, and it's dry by the time I'm ready to leave work. Try doing that in your tweed or cotton clothes.
South Korea in the summer is the same. Today we hit 37 degrees celcius with crazy humidity. Just a 11km ride at 30km/h had me drenched in sweat. It would be foolish to wear anything but synthetic cycling clothes.