When creating my nuke scripts I stream my main pipe through the 'B' side of the merge nodes. As you use nodes such as transforms, tracker, blur etc that makes your bounding box larger than your format it will also make your renders extremely slow.

This script will help make your renders faster by changing your BBox setting to B in Merge, Keymix, ChannelMerge and Copy nodes. Afterwards, it will tell you what has been changed.

Sometimes you may want to keep the BBox to A so be aware of that.

Select a group of nodes you wish to change and the script is launched under:

Edit>Node>Set BBox to 'B' side