[Rainrider]

I used this for testing:

local addon = ...
 
local UnitAuraCache = {} -- cached values are updated ten times per second and stored by unit+filter
local function UnitAura(unit, ...) -- 7.x to 8.x transience 2nd return value removed and only spellID argument is supported
    local retTable
    local index = select(1,...)
    if type(index) == 'number' then -- now only spellID is accepted
        retTable = {_G.UnitAura(unit, ...)}
        if retTable[1] then table.insert(retTable,2,'') end
    elseif type(index) == 'string' then -- lokup by spellName must be done on own by fetching all auras and then looking up by name
        local index, filter = select(1,...)
        local key = string.lower(filter or 'default')
        index = string.lower(index) -- lookup by spell name
        unit = string.lower(unit)
        local cache = UnitAuraCache[unit] -- fetch unit aura cache
        if cache and cache[key] and cache[key][index] and cache[key]['stamp'] == ("%.1f"):format(GetTime()) then -- cached values updated 10 times per second
            retTable = cache[key][index]
        else
            if not UnitAuraCache[unit] then UnitAuraCache[unit] = {} end
            if not UnitAuraCache[unit][key] then UnitAuraCache[unit][key] = {} else table.wipe(UnitAuraCache[unit][key]) end
            cache = UnitAuraCache[unit][key]
            local n = 1
            local name
            repeat
                retTable = {_G.UnitAura(unit, n, filter)}
                name = retTable[1]
                if name then
                    table.insert(retTable,2,'')
                    cache[string.lower(name)] = retTable -- there is no table.copy
                end
                n = n + 1
            until not name;
            cache['stamp'] = ("%.1f"):format(GetTime())
            if cache[index] then
                retTable = cache[index]
            else
                retTable = {nil}
            end
        end
    end
    return unpack(retTable)
end
 
local function GetAura(unit, ...)
    local index = ...
    if type(index) == 'number' then
        local name, texture, count, debuffType, duration, expirationTime, caster, isStealable,
            nameplateShowPersonal, spellId, canApply, isBossDebuff, casterIsPlayer, nameplateShowAll, 
            timeMod, effect1, effect2, effect3 = _G.UnitAura(unit, ...)
        return name, '', texture, count, debuffType, duration, expirationTime, caster, isStealable,
            nameplateShowPersonal, spellId, canApply, isBossDebuff, casterIsPlayer, nameplateShowAll,
            timeMod, effect1, effect2, effect3
    elseif type(index) == 'string' then
        local auraName, filter = ...
        auraName = string.lower(auraName)
        for i = 1, 100 do
            local name, texture, count, debuffType, duration, expirationTime, caster, isStealable,
                nameplateShowPersonal, spellId, canApply, isBossDebuff, casterIsPlayer, nameplateShowAll, 
                timeMod, effect1, effect2, effect3 = _G.UnitAura(unit, i, filter)
            if not name then return end
            if string.lower(name) == auraName then
                return name, '', texture, count, debuffType, duration, expirationTime, caster, isStealable,
                    nameplateShowPersonal, spellId, canApply, isBossDebuff, casterIsPlayer, nameplateShowAll,
                    timeMod, effect1, effect2, effect3
            end
        end
    end
end
 
local function TestCached()
    local start = debugprofilestop()
 
    for i = 1, 1e6 do
        UnitAura('target', 'nothing')
    end
 
    print('Execution time (cached):', debugprofilestop() - start)
end
 
local function TestNotCached()
    local start = debugprofilestop()
 
    for i = 1, 1e6 do
        GetAura('target', 'nothing')
    end
 
    print('Execution time (not cached):', debugprofilestop() - start)
end
 
_G['SLASH_' .. addon .. '1'] = '/testaura'
_G.SlashCmdList[addon] = function(msg)
    msg = strlower(msg)
    if msg == 'cached' then
        TestCached()
    else
        TestNotCached()
    end
end

The changes I made to Edik's code were:
1. removed the toc check (tested on beta only)
2. local index, _, filter = select(1,...) - > local index, filter = select(1,...)
3. cache[string.lower(name)] = table.copy(retTable) -> cache[string.lower(name)] = retTable

On average the cached version is 4 times slower than with no caching.

The test is not fair at all as it does not compare only the caching! The aim here is just to provide a rough estimation.

Beware that GetTime() is cached by the client. This has side effects on Edik's implementation as well.

The GetAura function can be optimized further.

Please do not take the results as a contra cache statement. Caching may prove useful, but this depends a lot on the use case and the implementation.