Alternative to saying "Long pole in the tent"

Solution 1:

You could say that a person or issue is a bottleneck in the project.

Solution 2:

Merriam Webster defines the phrase as: "The most important issue or problem that prevents or slows progress, especially on a project. The factor that must be addressed before all others or that has the most far-reaching effects." William Safire wrote a column about the phrase and illustrated how it has two overlapping meanings. He defined one as “the central determinant, the pole bearing the most weight of the tent structure.” The second meaning--brought about by George W. Bush and Dick Cheney--is “the source of delay, in the sense of holding everything else up.”

The most blunt way of conveying the same meaning is to say "x is the biggest problem" or "x is the major issue." But if you wanted to be less direct you could say "x is the bottleneck (or choke point)" or "x is on the critical path (or road to completion)."

Solution 3:

X is on the project's critical path