Hi MYGz,
Performance should be ok for 3.5K and take minutes, not hours. When you modify cust group from UI AX initialize tax group, payment terms and schedule from it, so if you want to have same behavior, then you need something like:
static void Job(Args _args)
{
CustTable ct;
CustGroup custGroup = CustGroup::find('110');
ttsBegin;
select forUpdate ct
where ct.AccountNum=='FDA01';
ct.CustGroup='110'; //Current CustGroup is '180' which is to be changed to '110'
ct.initFromCustGroup(custGroup);
ct.update();
ttsCommit;
}