Terms vs Terms lookup filter

Elastic Stack Elasticsearch
1

As stated in Elasticsearch documentation:

When it’s needed to specify a terms filter with a lot of terms it can be beneficial to fetch those term values from a document in an index

Are there any recommendation on what is considered "a lot"? When should one switch to terms lookup?
10 terms? 100? 1000?

2

until the query response take seconds or node in the cluster imminent to show sign of problem.