[Phanx]

You can make that even more efficient by storing the player's GUID in a variable, and referring to that, instead of looking up the same value over and over again every time a combat log event occurs. The GUID isn't available before PLAYER_LOGIN on a fresh login, so you'll need to wait until then to populate the variable.

Similarly, in your OnUpdate function, you have this:

        local function onUpdate(self, elapsed)
                totalTime = totalTime + elapsed
                if totalTime >= updateInterval then
                        for k, v in pairs(damageTable) do
                                if k >= time() - 5 then

You could improve efficiency by looking up time() and subtracting 5 once per OnUpdate, instead of once per table entry per OnUpdate:

        local function onUpdate(self, elapsed)
                totalTime = totalTime + elapsed
                if totalTime >= updateInterval then
                        local t = time() - 5
                        for k, v in pairs(damageTable) do
                                if k >= t then

Also, it's kind of pointless to define your OnUpdate function, and then set it separately:

local function onUpdate(self, elapsed)
        -- stuff happening here
end
frame:SetScript("OnUpdate", onUpdate)

You can just define the function anonymously in the SetScript call:

frame:SetScript("OnUpdate", function(self, elapsed)
        -- stuff happening here
end)

You might also consider clearing out old table entries; otherwise after a few hours of play you'll have a pretty large table.

                        local t = time() - 5
                        for k, v in pairs(damageTable) do
                                if k < t then
                                        -- old, get rid of it
                                        damageTable[k] = nil
                                else
                                        -- within the window, use it
                                        totalDamage = totalDamage + v
                                end
                        end