[Phanx]

Creating a new OnUpdate function every time you run the command is fairly wasteful. You should also use the elapsed value passed to your OnUpdate function, instead of calling GetTime() repeatedly.

local pull, seconds, onesec

local frame = CreateFrame("Frame")
frame:Hide()
frame:SetScript("OnUpdate", function(self, elapsed)
    onesec = onesec - elapsed
    pull = pull - elapsed
    if pull <= 0 then
        SendChatMessage("Pulling!", "RAID_WARNING")
        self:Hide()
    elseif onesec <= 0 then
        SendChatMessage(seconds, "RAID_WARNING")
        seconds = seconds - 1
        onesec = 1
    end
end)

SlashCmdList["COUNTDOWN"] = function(t)
    t = tonumber(t) or 6

    pull = t + 1
    seconds = t
    onesec = 1

    frame:Show()
end

SLASH_COUNTDOWN1 = "/inc"