Switch your display to speed up older Macs running LeopardTo cut to the chase: Leopard, besides switching off your firewall in the upgrade, also switches your display preferences for colours displayed from “thousands” – if that’s how you had it set – to “millions”.

Since that’s a difference which can suck up huge amounts of RAM, swap memory and CPU/GPU (“huge” being relative to the amount you have available), it’s a hidden change which isn’t very welcome. (For those who argue below that it “won’t make a difference” – System Profiler says that thousands of colours is 16-bit colour, and that millions is 32-bit colour. (Try it. Refresh System Profiler – Cmd-R – after you change the settings in the prefs. 32-bit must take extra GPU time and memory to process; it’s got to mean more data shunted around the system. It can’t be “no difference”.)
So, to speed up your machine: go to System Preferences, choose Displays -> Colours, and change the setting from “Millions” of colours to “Thousands”. Voila! Faster machine. Except see below – the change doesn’t stick for me.

And now, the backstory: since upgrading to Leopard – this on a 1.67GHz, 1GB RAM PowerBook (vintage March 2005) with 10GB or so of free disk space – I’d been puzzled by quite how slow it was. Switching between applications took ages; the windowserver application seemed to be taking a huge amount of CPU. Nothing happened fast.

And then after reading John Gruber’s notes about anti-aliasing, I thought I’d pay a visit to see whether my display was set to the right anti-aliasing setting.

It was. But what I also discovered was that Leopard, during the upgrade, had switched my display preference from thousands of colours to millions. (Strike another one up to the power of serendipity.)

I’m pretty confident that I had it on thousands before, because the difference to me between 8-bit colour and 12-bit colour (which I think, without checking, is the difference between thousands and millions) isn’t worth bothering with. I don’t do artwork on my machine. I’m not a photographer. I’d rather have fast and approximate with pictures than slow and precise. And I’ve made that change in the past to speed up my machine.

What I don’t understand is why Apple makes this change in the upgrade. It’s a guaranteed way to make your machine seem slower than it was before. Which isn’t what you’d want to do, is it?

The results for me: machine is faster; screensaver goes away quicker; switching between applications is faster. Visual penalty: undetectable. Try it and see. Obviously, if you’re disappointed, you’re entitled to a full refund of your five minutes spent here.

Update: OK, I’ve now done three logouts (one after yet another really annoying system total freeze) and the change hasn’t stuck. Every time I log in it’s back to millions. That’s either screwed up – which is bad – or a bad decision by Apple. Either way, grr. And these complete freezes are really trying my patience.