Calling ArcGIS processes from python can be a wonderful time-saver; allowing the user to code a process to loop through hundreds of data-sets autonomously.
Most ArcGIS processes are only 32 bit, so can slow down significantly when working on large datasets; data has to be shuffled in and out of memory (RAM). However, newer ArcGIS 64 bit processes allow more memory to be addressed, potentially resulting in significant reductions in processing time.