It does sound like there may be a cache-related issue with Sparkle, although so far I’ve not run into this myself. Something Duncan will need to look into with your specific Sparkle project file/s.
However one thing anyone can do to help prevent any type of slowdown like this (unless your OS is unstable/needs maintenance), is to use a fast external drive and move all your assets used for building the site and, the Sparkle project file itself to that drive.
And by fast I mean at the very least a 2-drive RAID-0, preferably SSD-based with Thunderbolt connectivity. You can find several options here: www.macsales.com
Because I edit commercial videos I’ve already had this setup so this could be why I’ve never experienced any slowdowns since I’m not using a single-drive system and making it work overtime by having the OS, apps, working files (photos, video, graphics, etc) and my working-cache files live on the same drive.
Granted my specific setup would be overkill for authoring websites, but anyone who constantly works in creating anything visually-based - like websites - should be using an external RAID to boost system and app performance and prevent or eliminate slowdowns such as this.
I still think Duncan should look at these issues and see if in fact there’s something app-related causing this, but regardless I can’t stress enough the benefits of having all your working files live on an external RAID. If you’re unsure which setup fits your rig and budget the guys at OWC (macsales.com) can guide you.