that one is for some access buffers and not actual temp tables?

No temp_buffers is for temp tables and materialized CTEs. Maybe you are confusing it with shared_buffers?

I am trying to say that your link says it is for access buffer for accessing temp table, and it doesn't say actual temp table is stored in that buffer and not flushed on the disk.

Huh, I think you're right... but it's still slower! I've definitely measured this effect in practice.

Just spitballing here — I think the difference might come from where the metadata required to treat the table "as a table" in queries has to be entered into, and the overhead (esp. in terms of locking) required to do so.

Or, perhaps, it might come from the serialization overhead of converting "view" row-tuples (whose contents might be merged together from several actual material tables / function results) into flattened fully-materialized row-tuples... which, presumably, emitting data into an array-typed PL/pgSQL variable might get to skip, since the handles to the constituent data can be held inside the array and thunked later on when needed.

Temp tables are per session, there shouldn't be any locking involved.

I believe the metadata about them is still written to various system catalog tables. Creating lots of temp tables will cause autovacuum activity on tables like pg_attr, for example.