I'd guess that that's something the server has to do since sync can only show the posts in the order the server delivers them (that's why if you hide a lot of posts you sometimes have to "open" the 2nd or 3rd site of the FrontPage)
If you have a post in a community with a weight over 20 then that post will only be sorted into your FrontPage after you loaded the site that post was on originally
Same as a post with a weight of 0.1 - it will always be on the bottom of your currently loaded page whatever that page ist