I think it would not actually be easy to test this. The massive combinations of hardware and software configurations in use out in the world make it nearly impossible to conclusively say one way or the other.

For instance consider the hypothetical of a service with a bug that increases its startup in certain circumstances. If Systemd triggered this bug and OpenRC didn't because of some default setting in each, perhaps a timeout setting, would you say OpenRC is conclusively better at start up time? Not really, they just got lucky that their default bypassed someone elses bug. Just off the top of my head other things that would probably cause hell in comparisons are disk access speeds, RAM bottlenecks, network load, CPU and GPU temp and performance etc.

You can perhaps test for specific use cases and sets of services, but I think this is more useful for improving each init system than it is as a comparison between them.

But then it wouldn't fit the "systemd = devil" narrative if it was actually tested and found out to be false lol