Stache will store a thumbnail screenshot, full-page screenshot and web archive for each page. In order to do this Stache needs to load the page you're bookmarking and then generate the associated files. This can be a relatively intensive and time consuming process.
On average a standard web page will take approximately 8 seconds to load before Stache can begin generating the associated files. As an example, complete processing time for http://getstache.com is 10 seconds.
Stache will not store web archives or full-page screenshots in iCloud (more info here). However, it will store a thumbnail for each bookmark in iCloud which are approximately 280kb on average.
On the Mac you import or Stache pages on, each bookmark will take up approximately 4MB of disk space on average (which includes the full-page screenshot and web archive).
Any other Mac's which you sync with will only store the page thumbnails locally and will not generate a full-page screenshot or web archive unless you manually choose to re-generate one on that device.
CPU and Memory Usage
When importing a large number of bookmarks this process can take several hours. You may also notice an increase in CPU usage and memory whilst bookmarks are processing. Mac OS X will make use of the resources available (memory and processing power) appropriately.