And if the situation is a little more urgent, we use the expression "It's high time" which means it's very important that this happens now - "It's high time we left - come on or we'll miss the train." Or again a parent might say to an older child - "It's high time you found a job, young man!"

So in summary -

"Let's go" is an informal suggestion or a polite instruction -

"It's high time" - is a formal statement that it is important that something happen soon or that something happen now.

And now - it's about time I finished this answer.

Now, the question is: does that qualify as a subjunctive mood? There is nothing about this structure on Grammar Girl’s site. I will take a close look at several reference books on grammar next week. Just for fun. But regardless of the result, I won't use it. Subjunctive mood is not in much use today. Only several common ones like "If I were you", "I would have ..."still in frequent use.