i find it may be too slow to edit the Wordpress site online,
is there any method to edit the posts offline? 
No
One word lives up to the name. Nice.
No is the answer. Wordpress runs off the database on the remote server.
But.
If you really want to get deep into it, rabbithole deep, you could install all of the elements on your local computer. Apache friends Xampp does it in one go. Then install Wordpress. This will give you a lighting fast editing and review, locally only. It is possible, (but I forgot how) to then upload everything to your remote server. I did that a while ago to get around lag times, but don’t remember all of the details.