direct writing into any table within any QAD could invalidate any support or warranties that you have with QAD, even if you are only updating a single table (with no risk of not updating other tables).
Having said that, if you dont have support or any warranties with QAD I would still say that you dont do any direct updates on data, however simple and straight forward they seem as you could never discount them from possible causes of errors further down the line!
Have you tried to analysis why the forcast upload is taking so long? Does it take long if you are typing them manually?