Comparison of excel files, cell by cell
You only ask about performance, but I want to point out anyway that there is a small error in the definition of file2RC: you used column instead of row. 

Now, let me get to the performance part. The thing making your code slow is that accesses to worksheets are expensive. Accordingly, they should be reduced to a minimum. 

Currently, you access each cell individually. A better approach is to load all on the same sheet in one go. You can do this by assigning the Value property, or better the Value2 property (Value causes problems with fields formatted as dates.), of the used range to a Variant variable. After that, the variable contains a two-dimensional array consisting of the values in the range. 

By comparing the values in corresponding arrays, you only need to access each sheet once to load the data and then for each time you find an error, in order to change the color.