I figure out where is the problem.
My scene contains many cProxy objects (bed, lamps, wash basin, stool, ...). After removing these objects, those two buttons works without delay. Even one cProxy objects causes some delay. More proxy objs. - more delay.
Probably I will never press these buttons again, because legacy is disabled automatically after conversion and PBR is ON. (As I remember it was necessary in older version converter/corona).
But still good to know what caused this issue :)