As Geometry Nodes experiences changes from time to time, the node tree will only work when it is opened in Blender 5.0 or newer versions (with the legacy version working all the way back in 3.1.0 but breaking with 5.1 and newer releases). 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.

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 disabling Notches or setting the Solver type to Manifold in the Geometry Nodes-modifier, the program should run smoothly as you tweak your shelf. You can simply turn the Notches back on or set the Solver type back to Exact once your happy with all other settings.