Speed of play is like driving a car - everyone thinks they are fine and it's everyone else who is at fault. This is a good thread - examining your own game and admitting some fault is commendable; we can all improve to some degree.
The well-hashed theory - and one I put some stock in - is that the average club golfer is trying too hard to emulate the pros. For obvious $$$ reasons, they take an ungodly amount of time to prepare to play a shot, especially around the greens. A combination of monkey see monkey do, and the conditioning by instruction experts on the importance of pre-shot routines and reading greens from 8 bloody angles has blown out playing times. Aiding and abetting this inclination is a general delusion that we are all better players than our scores suggest.
Someone in the group should take responsibility for hurrying up a tardy group. I was always taught the lowest handicapper is the unofficial 'captain' of a group so it's their role if no-one else is making noises. Enforce ready golf, send 2 players to the next empty tee whilst the other 2 putt out. Give directions on a lost ball hunt for other players to hit before joining the search. Drop hints when the search is fruitless or nearing a reasonable time of looking. Walk faster between shots and let everyone know that you are falling behind.
Anyone who has played in Scotland would have learnt that you can have a perfectly enjoyable round completed within 3.5 hours without running between shots. Players know there how to play without pissfarting around, and if they don't, there will be someone around to promptly let them know.
On a personal note, bloody Aimpoint should be banned. Hold 2 fingers up in front of your eye on the green and get an automatic escort off the premises. I occasionally play with a guy off 31 who I have not once seen reach 30 points. Every single putt, up go the fingers. Sleep inducing. Doesn't even begin until it's his turn to putt. That makes me part of the problem as he is such a decent bloke that I can't bring myself to tell him anything other than a general "we need to speed up a bit on the greens".
Other personal dislikes - marking and resting from within 4-5 feet, long pre-shot routines that only start when it's their turn to hit, any more than 1 practice swing, marking cards at the greens, leaving buggies at the front of the greens, not watching your ball when it's headed into trouble (I'm regularly guilty of this one), not calling a group through when they're up your arse and you are falling behind, and not knowing the rules r.e. hazards, lost balls, unplayables, etc. Not that it wastes much time but why do we pass the pin around like it's the holy grail? - put the bloody thing down somewhere out of the way and then stick it back in at the end.
In drafting this rant I can now answer the question posed by the OP - I contribute to slow play by not watching my errant drives as closely as I should, not knowing the rules as well as I should, and being too polite to tell a slow Aimpointer where he can shove his 2 fingers.