Slow Message Board the last few days...
I figured out the offending plugin. It was the Leira Letter Avatar plugin, which is also used here.
When I reactivated it, I didn't realized that you had to do a second save within the plugin settings to turn it back on. So when I noticed that a bunch of avatars were missing, I checked and saw I needed to save and turn it on. As soon as I did that, the site got slow again.
So I have turned it off...
Something must have happened recently that has caused a conflict with it on my server. It's too bad since I like that plugin. But I at least found the problem.
What you say is a bit confusing, because you told us that you did disable all plugins and no luck.
Anyways, Leira is something i use also without a single issue. But every site is different.