9.6 write time

I am looking at a postgres 9.6 on rh7 

I see that checkpoint_write_time is huge which looks quite strange as the average amount of data written is not that big.

For example in 5:30 hours today, data from pg_stat_bgwriter view, comparing values at 11AM and 4h30PM :

checkpoint_write_time 6986324

buffers_checkpoint 182447

so, to my understanding, it takes almost 2 hours to write 1.6 GB of data.

Can someone either correct my understanding, or shed some light on what can cause this ??

thanks,