When a program is performing a resource-hogging operation, you don't always have to leave it alone. If time is not of the essence, reader Prairie Moon recommends lowering its priority in Task Manager to keep it from slowing everything else down.
Sometimes the fastest way to finish a task in Windows, is to lower its process priority and then forget about it while you go and do something else, like heading off to browse Lifehacker. I mean, isn't that better than waiting for your favourite photo editor to finish batch converting a bunch of pics?
I did it this way, using IrfanView as an example. Once the conversion is underway, right click the Taskbar, choose Task Manager, find i_view32.exe, right-click, Set Priority to Below Normal or Low, click Yes, press ESC to get out of Task Manager. You can also do this for specific programs by creating shortcuts that start them in low priority mode. For example, with IrfanView the shortcut target would be %windir%system32cmd.exe /c start "IrfanView" /low "C:Program FilesIrfanViewi_view32.exe.
What all this does, is force IrfanView to settle for less of your system's attention and thus not slow you down when some other task needs to be right in front of you and getting its proper share of your computer's resources. It doesn't necessarily slow down the task that got sent to the back of the line, at least not by much; it merely forces it to take a backseat and not interrupt when other apps need to be front and centre.
There is also a subjective speed boost, which relates or dates back to the adage that a watched pot never boils, or 'a watched clock never gets to 5:00. So go on about your browsing business or whatever else, forget about your batch converter while it does its donkey work in the background, and it's guaranteed to feel like a lot less time.