Okey, I think I solved at least a part of the problem. First of all: if the plugin slows down the page, check your settings! Make sure you have the right applicationId and applicationSecret.
But now the real pain. Sometimes I HATE PHP because it has no strong typing.
File 'facebook-comments-functions.php' contains a method fbComments_combineCommentCounts($value)
// Return the cached comment count (if it exists)
if (get_option("fbComments_commentCount_$xid")) {
return fbComments_getCachedCommentCount($xid, $value);
}
The problem is it does not check for a proper NULL value. If the database contains a row with value '0', if doesn't work.
// Return the cached comment count (if it exists)
if (get_option("fbComments_commentCount_$xid") !== null) {
return fbComments_getCachedCommentCount($xid, $value);
}
This at least solves a part of the problem. Loading a single post is now a lot faster, but the overview pages are still a little bit slow...