lol exactly my point , .... kakashi trained sasuke's speed, and kakashi hasnt shown any out of the ordinary speed, when he was running from kakuzu's hearts, hidan was keepin up with him...
yes he shown lol i'll show you where it's stated
You must be registered for see images
and from tobi
You must be registered for see images
you talked about kakashi vs kakuzu+ hearts and hidan ,that was not 1on1 .and saved shika team.
You must be registered for see images
and
You must be registered for see images
the only part where kakashi got difficulty against kakuzu was in the end of fight.but kakashi's strategy was to gain time for shika give blood to hidan.
You must be registered for see images
kakashi didn't have to waste chakra for use kamui against kakuzu but if shika would be a bit later ,he could use it and kill kakuzu despite to go hospital

kishi did it to show us that kakashi could kill kakuzu .it's not an hazard.
You must be registered for see images