[Lag123]

Best place for optimising here is

Remove the

if not debuffList then return end

because you know that it exist
If you really want to check if the debuffList exists do that before register the event.

And localize the UnitDebuff function outside the SearchAuras function

local UnitDebuff = UnitDebuff

Not huge things but a small performance boost

local UnitDebuff = UnitDebuff
 
--get db via VARIABLES LOADED and saved variables
local debuffList = DB.debuffList or {}
 
local function SearchAuras(self,event,unit)
    local name, rank, icon, count, debuffType, duration, expirationTime, unitCaster, isStealable, shouldConsolidate, spellId
    for i=1,40 do
        name, rank, icon, count, debuffType, duration, expirationTime, unitCaster, isStealable, shouldConsolidate, spellId = UnitDebuff(unit,i)
        if not name then break end
        if not debuffList[spellId] then
          debuffList[spellid] = {name = name, rank = rank, icon = icon, debuffType = debuffType, spellId = spellId}
        end
    end
end
 
local auraCheck = CreateFrame("Frame")
auraCheck:SetScript("OnEvent",SearchAuras)
auraCheck:RegisterEvent("UNIT_AURA")