In most cases, insertion into a GIN index is slow due to the likelihood of many keys being inserted for each value. So, for bulk insertions into a table it is advisable to drop the GIN index and recreate it after finishing bulk insertion.
Build time for a GIN index is very sensitive to the maintenance_work_mem setting; it doesn't pay to skimp on work memory during index creation.
The primary goal of developing GIN indexes was to create support for highly scalable, full-text search in PostgreSQL, and there are often situations when a full-text search returns a very large set of results. Moreover, this often happens when the query contains very frequent words, so that the large result set is not even useful. Since reading many tuples from the disk and sorting them could take a lot of time, this is unacceptable for production. (Note that the index search itself is very fast.)
To facilitate controlled execution of such queries GIN has a configurable soft upper limit on the number of rows returned, the gin_fuzzy_search_limit configuration parameter. It is set to 0 (meaning no limit) by default. If a non-zero limit is set, then the returned set is a subset of the whole result set, chosen at random.
"Soft" means that the actual number of returned results could differ slightly from the specified limit, depending on the query and the quality of the system's random number generator.