External File Field

[Mark]

We have around 10million documents that are in our index and about 10% 
of them have some extra "statistics" that are calculated on a daily 
basis which are then index and used in our function queries. This 
reindexing comes at the expense of doing multiple joins in DIH so I am 
thinking it may be faster to precompute these values and use external 
files rather than have to re-index 10% of our corpus daily. How many 
external file fields could one use before it becomes "too many"? Is this 
a valid use case or am I trying to fit a square into a circular hole?