Video replay lends itself to the top league most, largely because the camera's are already there, cost a fair bit and need a camera man to, well, man them, and an editing team to throw together the replays in seconds. There are camera's/camera men at lower league games but its usually limited, massively so, and they'll edit the footage later on, not instantly.
If anything it needs trialing in the prem league u21 league and implementing in the main league and frankly this is one of those things that the premier league teams who make a killing could and should put money into the FA to pay for it to trickle down into the other leagues.
It simply wouldn't slow down games though, it would speed them up. Every time you get a ref stopping play at a corner and having a stupid talk with two players pulling each others shirts, replays, they'll give away penalties if they get caught so they'll stop doing it, less fouls and less ref's giving lectures.
The huge decisions often take ages anyway, I forget which freekick it was, during the West ham game this week or last week, it was a full two minutes from freekick to taking the freekick. Football is constantly stopped, and TWO whole challenges isn't going to ruin a game. Honestly the 4th official runs a tablet to the ref, the ref runs to him, max 10 seconds after a flag, 30 seconds max to see replays(I think thats the rule in american footie) often takes less than that, if the ref can clearly see the decision was wrong he'll overturn it, if not, he won't, simple as that.
I'd prefer a 5 minute longer game(which would be rare) than game after game with the wrong result. It would have cost Arsenal, but 30 seconds and QPR get 2 more points with a correct decision in game, and it was a goal so there was a noticeable delay till start of play anyway.
Fact is, if everyone gets caught for every shirt pull, for every dive, for every goal, if the players feel they are being treated fairly there will be a hell of a lot less complaining, and there will be a hell of a lot less cheating and fouls to start with as players learn they can't get away with it.