Slow Query, many tmpfiles

Hello, some simple query are very slow since a few days, no significant
changes are done the last days. I look for disk-io, swap, looked tables,
all seems to be good.

What I have found is this:

postgres=# SELECT temp_files AS "Temporary files"
, temp_bytes AS "Size of temporary files"
FROM pg_stat_database db;
Temporary files | Size of temporary files
-----------------+-------------------------
0 | 0
0 | 0
0 | 0
0 | 0
13542 | 5313912899891
(5 rows)

postgres(at)dolly:~/9.6/main/base/pgsql_tmp$ ls -la
total 20
drwx------ 2 postgres postgres 16384 Feb 3 02:25 .
drwx------ 8 postgres postgres 4096 Jul 26 2017 ..

Where does the 13542 tmp files are come from?
How can I delete that?
Where are they stored?