I think I found a work around to this problem. I noticed that the lagging happens on blog pages that have multiple entries that utilize thumbnails. When you are in the blog editor the left hand frame shows a lists of all the posts and loads all the thumbnails as a visual aid to help differentiate between each post at a glance. I find that the lag is caused by the editor loading all these thumbnails.
My work around is to click on the blog you wish to edit. The left frame shows all the blog posts and the right panel will display a web-page summary that can be seen online. Rather than select which blog post you wish to edit using the left frame click the "Pages" back button. It'll cycle back to the home menu but your summary page will remain in the right frame. Find your blog post through the right page and edit it from there.
If you are creating a new blog post, use the left frame to create a new page to the point where you can click out to "Pages" and still edit your new post.
I have found that this mitigates the lag issue. I hope that Squarespace implements a means of turning off thumbnails in the left frame because this appears to be what is causing all the slow down.