[DahkCeles]

I've found there's two sources of delay, especially in Classic: (a) asynchronously querying the localized spell name; and (b) waiting for the spell book to be available at PLAYER_LOGIN.

I quickly wrote the snippet below to convert spellIDs into localized names, but not to check the spell book until at least PLAYER_LOGIN and possibly later if asynchronous queries return info even later. This snippet also deletes non-existing spellIDs, which could happen for classic vs. retail differences.

If you are creating click-cast frames for use during combat, then you should change this to do as much as possible at PLAYER_LOGIN even if some spells are still slow loading -- its possible the player is logging in during a raid fight after the game crashed, and they will need to rejoin the fight on a best-effort basis before combat lockdown kicks in.

local spells = {1459, 23028, 1008,  604}  -- mage buffs
local spellsLoaded = 0
 
local function updateSpells()
  if (spellsLoaded == #spells) then
    for __, spellName in ipairs(spells) do
      if (GetSpellInfo(spellName)) then
        -- doSomething()
      else
        -- doSomethingElse()
      end
    end
  end
end
 
local playerIsLoggedIn = false
local f = CreateFrame("Frame")
f:RegisterEvent("PLAYER_LOGIN")
f:SetScript("OnEvent", function()
  updateSpells()
  playerIsLoggedIn = true
  f:RegisterEvent("LEARNED_SELL_IN_TAB")
  pcall(f.RegisterEvent, f, "ACTIVE_TALENT_GROUP_CHANGED") -- retail only
  f:SetScript("OnEvent", updateSpells)
end
 
local i = 1
while spells[i] do
  local entry = i  -- This local reference avoids race conditions between the loop and async queries.
  if (C_Spell.DoesSpellExist(entry)) then
    local spell = Spell:CreateFromSpellID(entry)
    spell:ContinueOnSpellLoad(function()
      spells[entry] = GetSpellInfo(spells[entry])
      spellsLoaded = spellsLoaded + 1
      if (playerLoginHappened) then
        updateSpells()
      end
    end)
    i = i + 1  -- moves to the next spell
  else
    -- The spell doesn't exist; maybe because of classic vs retail?
    tremove(spells, i)  -- shifts the next spell forward
  end
end