Well you just about have done everything wrong. See: How do I setup the QODBC Optimizer? Where are the Optimizer options? 

The best way is to let the QODBC work correctly as per my recommendations:

and then run your reports. The first time you run a report, it will take a little longer to update sync, but if you refresh the report or run another report off the same tables, Crystal Reports will work off the opimized tables.

This query:

SELECT Item.ParentRefFullName as ParentItem,
AVG({fn CONVERT(InvoiceLine.TxnDate, SQL_DATE)} - {fn CONVERT(BillItemLine.TxnDate, SQL_DATE)}) as AvgDaysHeld
FROM BillItemLine NOSYNC, Item NOSYNC, InvoiceLine NOSYNC
WHERE BillItemLine.ItemLIneItemRefListID = Item.ListID
and BillItemLine.ItemLIneItemRefListID = InvoiceLine.InvoiceLineItemRefListID
and BillItemLine.TxnDate >= {d'2007-01-01'}
and InvoiceLine.TxnDate  >= BillItemLine.TxnDate
and Item.ParentRefFullName is not null
GROUP BY Item.ParentRefFullName

will take a long time to process as it has to read every BillItemLine and InvoiceLine to determine the average days held. To speed up the report you need to limit the date range to exactly what you really need, for example change:-

and BillItemLine.TxnDate >= {d'2007-01-01'}

to:-

and (BillItemLine.TxnDate >= {d'2008-01-01'} and BillItemLine.TxnDate < {d'2008-07-01'})

Due to a bug within QODBC v8.00.00.242, I've had to use this evaluated function to process the dates correctly when using the QODBC optimizer:

({fn CONVERT(InvoiceLine.TxnDate, SQL_DATE)} - {fn CONVERT(BillItemLine.TxnDate, SQL_DATE)})

When this is corrected by the QODBC Software Engineer, you will be able to use:-

(InvoiceLine.TxnDate - BillItemLine.TxnDate)

which will be MUCH faster!!!!

In the meantime, you can use the DoCmd.TransferDatabase command to transfer the BillItemLine, InvoiceLine, and Item tables into MS Access and process the query there instead.

See: Can I use DoCmd.TransferDatabase command in MS Access?