What is the measure of how difficult it is for electrons to flow through a material?

You are probably looking for an answer like 'resistance' or 'resistivity', but these answers would be incorrect, because your supposition ('difficulty of flow') is incorrect .

Resistance is the opposition to the drift of an electric current through a material. It's determined by the resistivity of that material, as well as by the length and cross-sectional area of that material.

This opposition, however, is not a measure of the 'difficulty' of charge movement through the material. Rather, it is a measure of how well a material can support current due to the quantity of charge carriers available within the material.

It's important to understand, for example, that insulators don't 'block' or 'make it difficult' for electric current, they simply don't have sufficient charge carriers available to supportelectric current.