Some anecdotical evidence of mine. I tend to kill -9 Firefox or derivatives before system, or browser updates, to reliably get my tabs and cookies (for selected sites) back, without the need for any extensions.

Usually I'm doing that from within htop, or btop++. Under systemd that is slow, the process-tree of FF takes several seconds to vanish.

That felt very wrong. I increased the update frequency of htop and btop++ to 200ms (usually they poll/actualize/redraw at 2 seconds only) to investigate.

Then I retested that with Runit/S6(6) on the same systems.

Magic! The process-tree is instantly gone! And if you only SigHup it, it instantly reappears. BAM! BAM! BAM!

This applies to all sorts of process-trees also, not only FF.

Compared to that systemd feels like a sloth.

Yes, Yes, I did that under several different distros, initially AntiX, recently "init diversity edition"(Debian derivative optimized for 'live-booting', running from RAM, in all sorts of 'Frugal' installs), some Arch-derivatives, sometimes 'riced' to the max, and default Debian, just to be sure.

Over several years. Initially on a Core-i7 640LM with only 8GB RAM, more recently on Core-i5 7500t, and Core-i7 7700t with 32GB RAM.

Verdict: systematically slo(w)thified.

Do you think this is any different on more modern systems? Fear not, it gets worse with more cores!1!!