Relation between FIR filter's transition width and phase delay
In this answer I gave two well-known heuristic design formulas relating the filter order of (linear-phase) FIR low-pass filters to the transition bandwidth. According to both formulas, the transition bandwidth is inversely proportional to the filter order. Since the phase delay of a linear-phase FIR filter equals $M/2$, where $M$ is the filter order, the phase delay $\tau_p$ and the transition bandwidth $\Delta\omega$ are also inversely proportional to each other:

$$\Delta\omega\propto \frac{1}{\tau_p}$$