I've seen this site improve over the time I initially wrote this. The speed depends on your choice of markdown flavored syntax, which I think is neat that it lets you choose. This along with the ability to choose your theme and link with just about any online service, makes Dillinger very powerful markdown editor for the web.
JBT's Markdown Editor
This is a very fast but very minimal editor hosted on Github pages. If you want raw speed and minimalism and only need the ability to edit pure markdown (no synchronizing with online services), then look no further. There's not much to say other than it just works.
After about fifty lines or slow, you really do notice the slowdown with this editor unfortunately. They use jquery.markdown-0.2.js. At first it's clearly not visible, but it doesn't take long at all for it to become almost unbearable. After not long you'll be waiting a good 10-15 seconds after each couple characters you type for it to refresh.
Pilwon's Markdown Editor
If you prefer Github's markdown flavor and you only want to synchronize to Google Drive, then this is exactly the editor you want. It only can do those options, but it does them well.
They use a custom forked version of PageDown. The same one used by StackOverflow. Unless you're writing very large documents, you won't notice any slowdowns. It combines a WYSIWYG and Markdown experience.
|Editor||Speed||Google Drive||Dropbox||Github||Open Source|
|JBT's Markdown Editor||fast||yes|
|Pilwon's Markdown Editor||fast||X||no|