I'm not entirely sure when it happened, but suddenly all my locally-hosted WordPress sites are crazy slow (saving a page takes ~1-2 minutes). As soon as I move it to any external host, they're once again lightning quick, leading me to believe it's nothing to do with the WordPress site itself.
I realise this is quite a vague question, but how can I debug this? There's a good 40 or so sites hosted on MAMP, but I didn't think extra sites would slow it down if they weren't actively in use.