At the software companies where I have worked, we leave off the mention of a "tent" and simply talk about certain tasks or projects being "the long pole". It always refers to the project which will take the most amount of time, or rather will extend the longest into the future before completion. I thought this was a reference to how such projects appear on the overall GANTT chart of a software release plan - a project which can only be done serially appears as a single long "pole" on such a chart, extending into the future. But in light of the information presented here, it seems likely the phrase's origin is actually as a corruption of an original tentpole-based phrase.
But back to the OP question: what other phrase or term could be used to describe a person or issue which is taking the most time on a project or preventing progress on a project?
It's imprecise and has an unwelcome connotation, but the phrase elephant in the room would likely work in most cases. It has a connotation of there being a reluctance for people to speak of it, but in all other ways it seems appropriate: it is an issue (or person) that must be dealt with in order to move forward, or it is something that is taking up the most space in the room and therefore must be treated as a priority.
Specifically for issues which are preventing progress, a common term in software development is to call them blocking issues, for the obvious reason. This term would not generally be used for issues which merely are taking a long time, and would also generally not be used for people. You wouldn't call a derelict developer a "blocking engineer", though you might call their continued contributions a blocking issue!