In NT and Windows 2000, parallel ports do not use IRQs but are thread-based. The port is polled at background priority. This eliminates hardware conflicts with devices that want to use IRQ 7 or 5, ports that DOS and Win3.x used for the parallel port interrupts. If an application is running very slow due to large print jobs, you can reduce the thread priority of the port print function by adding the following registry entry. You can also use the this registry tip to increase priority for printing:

WINDOWS NT 3.51
Hive: HKEY_LOCAL_MACHINE
Key: SYSTEM\CurrentControlSet\Control\Print
Name: PortThreadPriority
Type: REG_DWORD
Value: Thread_Priority_Below_normal

WINDOWS NT 4.0
Hive: HKEY_LOCAL_MACHINE
Key: SYSTEM\CurrentControlSet\Control\Print
Name: PortThreadPriority
Type: REG_DWORD
Value: -1
where the default value in NT 4.0 is 0, which equals THREAD_PRIORITY_NORMAL. The value for THREAD_PRIORITY_ABOVE_NORMAL is 1 (increase parallel port's priority) and THREAD_PRIORITY_BELOW_NORMAL is -1 (decrease parallel port's priority).

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