Why projects slow down —
what really derails them

A project that slows down is almost never the victim of a single incident. It's an accumulation of small signals, often visible months in advance — but rarely taken seriously before it's too late.

The numbers leave little room for doubt

Bent Flyvbjerg's database — more than 16,000 large projects spanning decades — is unambiguous: 91.5% of megaprojects fail to meet either budget or schedule. His "iron law" of megaprojects fits in one sentence: over budget, over time, under benefits — over and over again.

This isn't unique to infrastructure. A McKinsey study with Oxford, covering more than 5,400 IT projects, shows large projects run 45% over budget on average and 7% over schedule, delivering 56% less value than predicted. Every additional year of duration adds 15% to the cost overrun.

PMI, for its part, measures a 13% project failure rate in 2025 — a slight increase from 2024. And BCG finds that more than two-thirds of large technology programmes won't be delivered on time, on budget, and within scope.

It's not a skills problem

The most common reflex when a project slows down is to look for someone to blame: an overwhelmed project manager, a poorly trained team. In practice, that's rarely the right explanation.

A project manager who centralises too many decisions ends up becoming the bottleneck themselves: if every call has to go through them, the project moves at the speed of their calendar — not the team's. It's the same mechanism described in our article on decision rights: a poorly calibrated role slows everyone down, regardless of how skilled the people involved are.

4 signals that foreshadow a slowing project

1. The project manager becomes the bottleneck

Every decision, even minor ones, escalates to a single person — the project's pace becomes the pace of their calendar.

2. Scope grows without ever being formally approved

34% of projects experience scope creep (PMI) — each addition seems minor, but the accumulation changes the nature of the project without anyone explicitly deciding so.

3. Every extra month costs more than the last

Cost overrun worsens with duration (McKinsey: +15% per additional year) — a delayed project doesn't cost linearly more, it costs increasingly more.

4. Success is only measured at the end

Without a checkpoint where drift can be objectively flagged, the warning signal often only arrives once it's too late to correct course without damage.

The question executive committees should be asking

Not just: "Is this project on schedule?"

But rather: "On this project, who has the right to say no to a scope extension — and do they actually exercise it?"