[woot]

You're looping (up to) 15 times per second through 100 players, and that for every player. Remove that .. thing from OnPlayerUpdate.

Here's a really simple fix. 10 seconds should be more than enough.

pawn Code:

// OnGameModeInit
SetTimer("UpdatePlayerCount", 10000, false);

// OnPlayerSpawn
TextDrawShowForPlayer(playerid, players);


// Note that you don't have to re-show the textdraw if it gets updated.
function UpdatePlayerCount();
public UpdatePlayerCount() {
    new
        str[ 10 ];
    format(str, 10, "%d/100", GetOnLinePlayers());
    TextDrawSetString(players, str);
}