"This is no time to slack off," is talking about right now, this very moment. Maybe our boss is in the office, so we cannot take a break we did not earn, act distracted or lazy - because this is no time to slack off. While, "There's no time to slack off," means in general. Let's say we have a project due or if we have a business meeting next week. From now to next week we cannot get distracted or get lazy, because there is no time to slack off if we want to meet the deadline.