Long-term opportunity cost - emergency fund before starting investing vs. combination of investing and saving simultaneously

For a young investor with no debt, no kids, no wife, how do you generally think about prioritizing an emergency fund versus continuing to invest?

For example, suppose someone needs around 9–12 months to build a 3-month emergency fund if they temporarily reduce investing substantially. Alternatively, they could split their savings between investments and cash, but the emergency fund would take considerably longer to complete.

Over a 35-year investment horizon, is the opportunity cost of temporarily reducing investments for \~1 year generally significant?

I’m particularly interested in the trade-off between:
building the emergency fund as quickly as possible while investing only a small amount, and
continuing substantial monthly investing while building the emergency fund more slowly.

I understand this depends on individual circumstances; I’m mainly interested in the general portfolio/risk-management reasoning behind the two approaches.