No. The rule is still to answer all messages within 24 hours of receiving them.
Check to see if there are messages in your spam folder. Sometimes that can affect your rating which is not fair, and if you do find that is the case, then write to customer service and explain this and ask them if they can reset your response rate.
Also, the stats for your Gig are calculated for 60 days - so, there may have been a message or two that you missed over 60 days ago and is now affecting your response rate.
If none of the above apply, do message customer service and ask them to check your response rate because you think it may be off - they will do that and let you know where you may have missed a message or they may find it is a bug and reset it for you.