STP - Time to select root bridge

When applying Spanning Tree Protocol (STP) to a Layer 2 topology, the process of selecting a root bridge may take varying amounts of time depending on several factors such as the number of switches in your network, the speed of your links, the current load on your network, and the specific STP enhancements you have enabled (like RSTP, UplinkFast, or BackboneFast). It’s normal to see some variability in convergence time. To speed up the convergence time, you can consider enabling RSTP or tuning your STP timers.

https://networklessons.com/cisco/ccie-routing-switching-written/introduction-to-spanning-tree