On Facter 2
facter -v
2.5.7
facter --external-dir /opt/puppetlabs/facter/facts.d
SLOW FACT
...
slow => true
On Facter 3
On Facter 4
If we use the Ruby API:
Facter.to_hash and Facter.value('fact_name'), both load external facts on all versions of Facter (2, 3, 4)