After the system's one time tasks (stage 1) are done, the system services are started up in parallel. The operating system's process scheduler takes care of having the services available as soon as possible.

One of the runit project's principles is to keep the code size small. As of version 1.0.0 of runit, the runit.c source contains 330 lines of code; the runsvdir.c source is 274 lines of code, the runsv.c source 509. This minimizes the possibility of bugs introduced by programmer's fault, and makes it more easy for security related people to proofread the source code.
The runit core programs have a very small memory footprint and do not allocate memory dynamically.

Simple scripts linked to the proper directory is all that's needed to bring a service up at boot, and everything is up and running quickly.

UNIX philosophy, easy to add new services, easy to manipulate, really fast.

Myself included. Development is no longer stalled.

Faster on older systems, especially those running on HDDs.