It's many times solved by telling the slow player they are slow, why they are slow and if no fix, uninvited. Explain that your group of guys require fast play. Anyone not being fast, out (as a rule, not because you're not a cool guy to be with).
We also have a 2 stroke penalty applied to the group that turns in the card more than 17 min behind the group before them. When you explain this to the slow player, that they are the reason others are getting dinged, most get and will either not play, or pick up the pace. (if they are not so self absorbed, meaning if they're too self absorbed.... out). On the other side, the other players also not to be so self absorbed to occasionally assist the slower guy, not all the time though.