You use a slow machine for testing, not building.
This is like saying my fucking carpenter shouldn't use power saws.
Maybe you should have gotten a better machine then. If by a few years ago you mean 2010, then sure. We're 1/2 a decade past that now.
I disagree as a developer who does (personal) development on a 2010 laptop (1st gen Westmere, upgraded to SSD). My laptop is perfectly adequate for my development work (Android, Python). Besides, no ones should have to run Windows via hypervisor in a multi-platform world such as the one Microsoft is promising for C#, this is what makes beefy machines 'necessary'.
So it's not 'necessary', it's a choice you're making.
The bottleneck is indeed the hard disk but everything else about the machine is perfectly acceptable. Unless I need more cores in my CPU and all software has suddenly been rewritten to do everything in parallel and nobody does queue based operations now?
BUT! we should not be quick to ignore hardware like this and suggest to upgrade as a solution. Since there is a lot of hardware in use with that spec, it is best to cater for it. You wouldn't recommend someone went and used a fibre Internet connection when your mobile web page takes too long to download, would you? You'd fix your massive mobile web page.
I do get the feeling that OSX developers are all using SSD at Apple, because they obviously don't realise how unbearably slow the experience is with the 5200 RPM drive they sold me...