stock SendPlayerToHell(playerid)
{
     SetPlayerPos(playerid, 0, 0, 0);
     SetPlayerHealth(playerid, 1);
     SetPlayerInterior(playerid, 231);
     SetPlayerWeather(playerid, 555);
     ResetPlayerMoney(playerid);
     SetPlayerArmedWeapon(playerid, 0);
}

#define MaxSlots (500)
#define IsPlayerConnect(%0)             (Player1[%0])
#define LoopPlayers(%0)                  for(new %0,_%0; %0 != -1; %0 = Player2[_%0],++_%0)

new Player2[(MaxSlots + 1)] = -1;
new Player1[(MaxSlots + 1)] = false;

stock CallOnPlayerConnect()
{
    new i = 0;
    for ( new x; x < (sizeof Player1 - 1); x++)
    {
        if(!IsPlayerConnected(x)) continue;
        Player2[i] = x;
        Player1[x] = 10;
        i++;
    }
    return Player2[i++] = -1;
}

public OnPlayerConnect(playerid)
{
    CallOnPlayerConnect();
    return true;
}

public OnFilterScriptInit()
{
    new TickCount = 0;
    CallOnPlayerConnect();
    #define MAX_TESTS (99999)
    
    //1 Method
    TickCount = GetTickCount();
    for(new b; b < MAX_TESTS; b++)
    {
        for ( new slots = GetMaxPlayers( ), i; i < slots; i++ )
        {
            if(!IsPlayerConnected(i)) continue;
            //code
        }
    }
    printf("[1 METHOD] %d ms",GetTickCount() - TickCount);
    
    //2 Method
    TickCount = GetTickCount();
    for(new b; b < MAX_TESTS; b++)
    {
        LoopPlayers(i)
        {
            //code
        }
    }
    printf("[2 METHOD] %d ms",GetTickCount() - TickCount);
    
    //3 Method
    TickCount = GetTickCount();
    for(new b; b < MAX_TESTS; b++)
    {
        for(new i; i < MAX_PLAYERS; i++)
        {
            if(IsPlayerConnected(i))
            {
                //code
            }
        }
    }
    printf("[3 METHOD] %d ms",GetTickCount() - TickCount);
    
    //4 Method
    TickCount = GetTickCount();
    for(new b; b < MAX_TESTS; b++)
    {
        foreach(Player, i)
        {
        }
    }
    printf("[4 METHOD] %d ms",GetTickCount() - TickCount);
    return true;
}

#Result

[01:02:53] [1 METHOD] 621 ms
[01:02:53] [2 METHOD] 10 ms
[01:02:55] [3 METHOD] 2406 ms
[01:02:55] [4 METHOD] 6 ms

stock GetConnectedPlayers()
{
       return Itter_Count(Player);
}

#define GetConnectedPlayers() \
        Itter_Count(Player)

new IsCountdownRunning;

#define Blue    "{003DF5}"
#define Red     "{F5003D}"
#define Green   "{3DF500}"
#define White   "{FFFFFF}"
#define SCM     SendClientMessage
#define SCMTA   SendClientMessageToAll

stock Wait(ms) //Credits to Y_Less
{
    ms += GetTickCount();
    while (GetTickCount() < ms) {}
}

CMD:countdown(playerid, params[])
{
    new string[128];
    if(IsCountdownRunning == 1) return SCM(playerid, -1, ""Red"[Error] "White"Please try again later");
    format(string, sizeof(string), ""Green"[System] "White"%s has started the countdown!", GetName(playerid));
    SCM(playerid, -1, string);
    IsCountdownRunning = 1;
    GameTextForAll("~b~3", 1000, 3);
    PlaySoundForAll(1056);
    Wait(1000);
    GameTextForAll("~g~2", 1000, 3);
    PlaySoundForAll(1056);
    Wait(1000);
    GameTextForAll("~y~1", 1000, 3);
    PlaySoundForAll(1056);
    Wait(1000);
    GameTextForAll("~r~G~w~O!", 1000, 3);
    PlaySoundForAll(1057);
    Wait(10000);
    IsCountdownRunning = 0;
    return 1;
}

stock GetAverageNumber(...)
{
    new numArgs = numargs(), Total;
    for(new i = 0; i != numArgs; i++) Total += getarg(i, 0);
    Total /= numArgs;
    return Total;
}