It depends on how you play. You can finish a scenario with a bronze, silver or gold medal, depending on how fast you do it. If you try for this in games anyway, then you won't run into any problems. If, however, you want to explore the whole map, look in every corner and take your time, you may find yourself not completing the mission in time.
The pace is brisk, but this does go with the storyline. You have to move and you can't sit around doing little (in terms of the story). It's not as bad as some messages I saw make it sound, though, and it's part of the challenge. It's a strategy title, not an exploration game.
