why select @@session.tx_read_only take long time

[h h]

i find in my mysql slog log ,i don't understand ,why select @@session.tx_read_only take so long time,any one can help?,and mysql version is
5.7.12

# Time: 2016-10-31T11:02:11.515814+08:00
# User@Host: vs_rw_venus[vs_rw_venus] @ [10.51.29.18] Id: 8338975
# Query_time: 18.900807 Lock_time: 0.000000 Rows_sent: 1 Rows_examined: 0
use venus;
SET timestamp=1477882931;
select @@session.tx_read_only;
# Time: 2016-10-31T11:02:11.515879+08:00
# User@Host: crm_rw_ucenter[crm_rw_ucenter] @ [10.117.209.99] Id: 8335677
# Query_time: 18.898368 Lock_time: 0.000000 Rows_sent: 1 Rows_examined: 0
use ucenter;
SET timestamp=1477882931;
select @@session.tx_read_only;
# Time: 2016-10-31T11:02:11.515932+08:00
# User@Host: crm_rw_ucenter[crm_rw_ucenter] @ [10.117.51.149] Id: 8330374
# Query_time: 18.873369 Lock_time: 0.000000 Rows_sent: 1 Rows_examined: 0
SET timestamp=1477882931;
select @@session.tx_read_only;
# Time: 2016-10-31T11:02:11.515996+08:00
# User@Host: grace_rw_grace[grace_rw_grace] @ [10.117.51.149] Id: 8338310
# Query_time: 18.872921 Lock_time: 0.000000 Rows_sent: 1 Rows_examined: 0
use grace;
SET timestamp=1477882931;
select @@session.tx_read_only;
# Time: 2016-10-31T11:02:11.516056+08:00
# User@Host: crm_rw_ucenter[crm_rw_ucenter] @ [10.117.209.99] Id: 8333928
# Query_time: 18.868223 Lock_time: 0.000000 Rows_sent: 1 Rows_examined: 0
use ucenter;
SET timestamp=1477882931;
select @@session.tx_read_only;