The importance of time is pretty relative. Think about the two and a half hours you spent watching Inception: there is clearly nothing better than doing just that, so it was well-spent. How about the five hours you spent taking Buzzfeed quizzes, though? That was, um, probably less well spent.
Time for Cobb is essential because every moment that he has not spent with his kids is time lost. Luckily for him, time in the dream world moves much more slowly. We get some crazy cross-cuts of the different times moving at different speeds, emphasizing both the simultaneity of events and the importance of every second.
Questions About Time
Did Cobb ever really grow old with Mal? Does time in Limbo have as much value as time in other dream levels or in the physical world?
What would it feel like being Saito? He's lived with himself (literally, with projections that are his subconscious) for forty or fifty years. What would it feel like coming back to an old life you hardly remember?
If you had five minutes of slow motion, what would you do?
Chew on This
Inception is a race against time for Cobb, who must make amends before it's too late. The emphasis on time, and watches, and slow-mo isn't all about literal dreamtime; it's about Cobb rushing against the clock to get back to his family.
Living as an "old man filled with regret" is all about living alone. Saito's stay in Limbo is far worse than Cobb's when he had his wife. Saito's regret is unshared and the regret that Cobb is trying to avoid stems from a future without his children.