Thanks for the quick reply...
Oddly enough, another smaller epub I have does it too, but to a much lesser degree. War For The Oaks epub (free from Tor) is 368Kb and the delays are smaller - around 10-12 seconds to open and only slightly sluggish responses when editing.
System : iMac 2.8GHz Intel Core 2 Duo, 4Gb RAM
OS : Mac OS X 10.5.8
Sigil idle, no ebook loaded, 0% CPU, 25Mb mem used
When opening War For The Oaks (368Kb), Sigil's CPU usage goes to 100% and its memory jumps from 25Mb used to 45Mb used then CPU usage settles at 0% once epub is loaded.
When entering carriage returns in the middle of a paragraph, CPU usage is about 25% but when deleting them, CPU usage hits 90-100% and it takes around 1s per deletion.
I then closed Sigil and reopened it before loading the 1.4Mb Deed of Paksenarrion epub. Activity monitor settled at 100% CPU usage and quickly reported Sigil (Not Responding) and memory usage climbed rapidly to 112.5Mb used. Once the epub was loaded, CPU settled back to 0%.
I am not sure if that degree of memory and CPU usage is what you would expect.