[SDPhantom]

This is what I've come up with.

local AsyncPairs; do
--  Local pointers make lookups faster
    local CallErrorHandler=CallErrorHandler;
    local debugprofilestop=debugprofilestop;
    local next=next;
    local table_remove=table.remove;
    local unpack=unpack;
    local xpcall=xpcall;
 
    local TableStack={};
    local TimerFrame=CreateFrame("Frame");
 
    local function TimerFrame_OnUpdate(self,elpased)
        local tbldat=TableStack[1];--   Pull from bottom of stack
        if tbldat then
            local tbl,key,limit,oniterate,onfinish=unpack(tbldat,1,5);--    Extract vars; range is specified since there can be embedded nils
            local start=debugprofilestop()-elapsed*1000;--  Offset with frame render time
            repeat--    Used instead of While to guarantee at least one iteration
                local val; key,val=next(tbl,key);-- Get next key/val pair
                if key~=nil then--  We have a key/val pair
                    local ok=xpcall(oniterate,CallErrorHandler,key,val);--  Dispatch OnIterate callback
                    if not ok then table_remove(TableStack,1); break; end-- OnError: Remove table from stack and exit
                else--  No more key/val pairs
                    table_remove(TableStack,1);--   Remove from stack
                    if onfinish then xpcall(onfinish,CallErrorHandler); end--   Dispatch OnFinish callback
                    break;--    Exit loop
                end
            until debugprofilestop()-start>=limit-- Checked at end of iteration; compare elapsed time to limit
            tbldat[2]=key;--    Store new key for next frame; if table was removed from stack, this is just garbage anyway
        else self:SetScript("OnUpdate",nil); end--  Disable if we have nothing on the stack
    end
 
    function AsyncPairs(tbl,limit,oniterate,onfinish)
        table.insert(TableStack,{tbl,nil,limit,oniterate,onfinish});--  Push to stack
        TimerFrame:SetScript("OnUpdate",TimerFrame_OnUpdate);-- Activate timer function
    end
end

Usage: AsyncPairs(Table, Limit, OnIterate[, OnFinish])
Limit is in milliseconds. OnIterate and OnFinish are callbacks with the later being optional.