The order list has a couple of possible performance issues, they are as follows:

1. Calculation of total ex. vat and total inc. vat involves doing lots of calculation for each order item. The reason is that this information is not stored in the order table. The same thing is probably done for each order item.
2. Temporary and active orders are stored in the same table (differentiated by is_temporary). This causes each SQL to have the is_temporary=0|1, which in turn causes the sorting to not use an index (the sorting index is also missing).
Splitting the data into two separate tables might help out here.
Note: It makes sense to wait for 4.0 to do this.