The first time ever I experience a serious lagging of the keyboard with Snow Leopard and Scrivener 2.3.1 (20025) on my Macbook 13".
The document is nothing special.
The lagging document contains PNGs in the maintext. There is no keyboard lagging in documents without images in the text.
If you have a lot of large image files in the text, or even a few, it will take a while to save the text file, so it may just be auto-save kicking in every two seconds after a pause.
All the best,
Keith