The Geometry Node tree is, for the most part, designed to have fairly good performance. There is, however, one feature which slows the computation down heavily because it employs a mesh-boolean operation. That feature is the notches you can carve into the edges of the wooden planks. By turning the notch_frequency down to 0.0 in the Geometry Nodes-modifier, the mesh-boolean operation will be bypassed and the program should run smoothly as you tweak your shelf. You can simply turn the notch_frequency back up once your happy with all other settings.

As Geometry Nodes is a fairly new feature in Blender's development and has experienced many changes in the recent releases, the node tree will only work when it is opened in Blender 3.1.0 or newer versions. Also make sure that you haven't previously opened and saved the file in an older version as that can permanently destroy the node tree. In that case you'd need to redownload the file.