we recommend all the time against slurping files in all at once unless really needed
Do we? How strange. Why?
Seems to me that slurping is a perfectly valid technique in the right circumstances, for instance, parsing command output or working with small to middling data sets. People have even gone to the trouble of writing modules to support the idiom.
In this case it would appear that ultibuzz has a data set of 5000000 numbers so slurping, as it turns out, would definitely not be appropriate.