Speed is a communication property
Adding people increases capacity only when the work separates cleanly. On ambiguous product problems, every new boundary creates another place for context to thin out and decisions to wait.
A small senior team can hold more of the problem at once. Product, design, and engineering trade constraints in real time instead of passing documents between functions.
Senior means close to the work
Experience is most valuable at the point of making. The people framing the architecture should see the operational details; the people shaping the experience should hear the technical constraints.
The goal is not heroics. It is a team with enough range to make sound decisions, enough trust to challenge one another, and few enough layers to keep moving.