Have you tried installing
Arch Linux on your 2009 iMac? It's a great exercise in needless, self-inflicted suffering, but if you really want to wring every last bit of performance from the hardware, there's realistically no better choice than Arch. ✌️
That's something many Linux users seem to think. But as with all things, what most people think is not always a reality. Think of Islam, billions of people who believe in something for which there is not the slightest evidence. Or think of current science, where the most highly educated people in existence often have opposing opinions in large groups on basic matters.
Arch Linux is on average not really faster than eg Debian:
https://www.phoronix.com/benchmark/result/cachyos_linux_benchmarks/f3dce626644e.svgz
In specific benchmarks you often see very large differences and for things like browsers, newer versions often have higher performance. But on average they are close together and you see that Clear Linux is the only one that can make a big performance gap. Sometimes the gap Clear Linux cuts is much bigger than what you see here. But Clear Linux is probably not going to be supported by the old CPU in your devices, but you can try it just to be sure.
So what are methods that do work to make older hardware snappier?
A first method is to install FreeBSD instead of Linux. FreeBSD often outperforms Linux on old hardware. For example, in the comprehensive Jetstream 2 as well as the WebXPRT 4 benchmark, FreeBSD scores 25% higher than the average Linux distro and on average similar to Clear Linux.
User Review of FreeBSD: 'I have been using FreeBSD as a development environment on several machines for work and personal projects - mainly NodeJS and MongoDB based. I chose FreeBSD as my environment initially because of its speed on older hardware, it allowed me to make use of older laptops and...
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I first tried different distributions of Linux, but I had trouble getting them to install or they did not perform on older hardware well at all. From my use of FreeBSD I have come to really really like it, I appreciate it's robustness and performance and I also like that I am using a direct descendant of UNIX.
A second method is to use a window manager instead of a full desktop environment. bspwm is possibly the fastest window manager currently in existence, just slightly faster than dwm.
A third method, if you're using a Linux distro, is not to choose a system that uses systemd. Devuan, MX Linux and Void Linux boot much faster than the typical systemd systems like Ubuntu, Mint, Arch, Fedora etc.
And a last obvious tactic is to use lightning fast apps like Thunar, Viewnior, mpv media player, Claws Mail, zathura document viewer, Atril document viewer, Evince, mate-terminal, Geany, Emacs, etc.
