The tone from my perspective is strictly serious. But not VERY important.
What’s the tone of this msg?We need 2 find a way 2 significantly reduce time you spend on ABC.Please could you try 2 identify why you spend so much time?Is it because the reporting prepared by ZZ is not correct, or you review in too much detail?
They've been asked by their manager to justify why you are spending more time than is usual on this account. So you need a face-to-face with whomever sent you the message. Get your ducks in a row first to explain the reason why this account / file / etc. needs a little more attention than the other accounts.
They are able to run reports on just about everything. When they ran a report and saw the extra time pop up on this account, it raised a red flag. They went to your manager - who had no answers and it put him / her in a bad spot.
So they went to you for an explanation. Like I said, have your ducks in a row - have all your facts ready - don't get defensive - just state what's going on.