Time constraints help build suspense because it gives the hero a slimmer chance of succeeding. The less time the hero has, the slimmer the chance. Every minute the hero loses dramatically increases the suspense for the reader. Many writers use time constraints in their story by setting a deadline or cut-off time for a task to be accomplished, and if it isn't accomplished in that amount of time something terrible is predicted to happen.
If the hero has a lot to lose it also adds to the suspense in the story. If the hero is willing to do anything to stop an event from happening, it demonstrates to the reader the urgency of the situation. The thing that is at stake is usually important and often affects many people, not just the hero. This helps the reader sympathize with the hero and want him to succeed.
Dilemmas are when the hero must make a choice between two outcomes. In most cases, either choice he makes will have a negative outcome in some way. The hero often has to chose the lesser of two evils. An examples of a dilemmas is when a hero can only save one person by letting another person die. Dilemmas often increase the stakes of the situation and take time for the hero to solve, both of which contribute to the suspense.
Foresight is when the author gives the reader a glimpse of what is to come in the story. Foresight lets the reader know what the stakes are and what the hero has to accomplish. Foresight also lets the reader know what trouble the hero is in before the hero even knows. This creates suspense because the reader can only watch the hero move towards the trouble without being able to do anything about it.
When the hero has to balance multiple problems, events or situations at once it makes the story more suspenseful. A story line that is made more complicated by adding conflicts and complications is suspenseful because it increases the odds that the hero won't succeed. This leaves the reader wondering exactly how they will succeed and compels them to continue reading.