What's hard to believe about Kono knowing rasengan? He had 2 and a half years to learn it. If Naruto could learn it in a short amount of time with his own training, I'm sure Kono could learn it in that amount of time with normal training.
So, Naruto had "
his own training" and Konohamaru had "
normal training".
Naruto had guidance, direction and a sounding board ("I can't get this quite right" - "OK, try this focusing on this symbol in the palm of your hand" as one example). He had someone who had mastered it able to check on his progress from time to time. He saw it formed several times, spread throughout his training.
Konohamaru didn't even as Ebisu for guidance or help, and there is nothing to suggest that he asked Kakashi either.
In fact, when Kakashi showed Naruto that
he new Rasengan,
that was a big revelation ("Wow, Kakashi knows it?").