There is nothing essentially about it - they have equal performance. Since the Intel docs specify that even an m32real and m64real being loaded from memory perform at exactly the same speed. Regardless of bus the CPU has been designed so that a float fetch or a double fetch as it relates to pushing values onto the FPU stack (keep in mind this only applies for pushing values onto ST(0)) operate at the same speed. If this were not true then the clocks would change.
while at the CPU level floats and doubles may have (essentially) equal performance,