private strand flush not complete in alert log

I consider myself as an "advanced beginner" to Oracle database administration so you may excuse if my question should be absurd ;-)

In one of our dev databases (Oracle 12c on Linux) we frequently (about once a minute) see

Thu Apr 07 10:42:24 2016
Thread 1 cannot allocate new log, sequence 71029 Private strand flush not complete
  Current log# 2 seq# 71028 mem# 0: +ORAREDO1/DEVEE072/ONLINELOG/group_2.269.854394251   Current log# 2 seq# 71028 mem# 1: +ORAREDO2/DEVEE072/ONLINELOG/group_2.269.854394251 Beginning log switch checkpoint up to RBA [0x11575.2.10], SCN: 4102856777 Thu Apr 07 10:42:27 2016
Thread 1 advanced to log sequence 71029 (LGWR switch)   Current log# 3 seq# 71029 mem# 0: +ORAREDO1/DEVEE072/ONLINELOG/group_3.268.854394253   Current log# 3 seq# 71029 mem# 1: +ORAREDO2/DEVEE072/ONLINELOG/group_3.268.854394253

We also had "cannot allocate new log" together with "checkpoint not complete". But seems that we got rid of this by increasing db_writer_processes from 2 to 4. (we did the change yesterday, so we still observing this)

According to MOS note 372557.1 :
"These [private strand flush not complete] messages are not a cause for concern unless there is a significant time gap between the "cannot allocate new log" message and the "advanced to log sequence" message."

In our case the time gap between this two messages regularly is 3seconds. Would anybody of you consider this as "significant time gap"?

Many thanks

Manfred

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