You can significantly improve the performance of ArcPro by building indices for your data.  RivEX will try to do this for you if it sees that your dataset is without a spatial index.  This process can fail (e.g. the file was schema locked by another application) so you may want to build these indices independently.

You can tell if your dataset has a spatial index by opening the attribute table of the the dataset and looking at the field header for the Shape field. If you see a small asterisk then it has a spatial index.

Performance can also be improved with attribute indices. Some of the RivEX tools have an optional processing task of adding an attribute index to the output dataset; this you can simply tick on. Alternatively indices can be built by calling the appropriate Geo-processing tool.

If your data is stored as a File Geodatabase FeatureClass then you do not need to build a spatial index, this is already handled by the Geodatabase but you will want to consider building attribute indices.  

WarningIt is strongly advised that all your data sits within a File GeoDatabase for the best performance

You can add spatial and attribute indices with standard geo-processing tools. More information about these tools can be found here.