I’ve seen a lot of posts online regarding this issue. I’m using the latest version of WordPress (5.0.3) and I have been experiencing slowdowns on the Post Preview. This seems to be related with the new Gutenberg editor. In my case, clicking the Preview button shows a white page that says “Generating Preview” for an annoying amount of time before the Preview is actually shown.
Though in your case the preview is completely blank, right? I think it’s most likely related. So we wait until the WP core devs fix this issue. In the meantime what I did was install a plugin that brings back the Classic Editor so I can use the Post Preview correctly.