[Analytics] Ingestion rate is 2-3 times slower with DEBUG logging level (new default)

Setting this ticket as Critical because DEBUG level is default logging level since build 1370.
The rate of degradation:

6.0.0-1388 and 6.0.0-1432 were tested with DEBUG logging level
And for 6.0.0-1460 I've manually switched it back to WARN

The degradation is all across the product. Query latency degraded as well

Comparison between 6.0.0-1432  (DEBUG) and 6.0.0-1460 (WARN):
Avg. ingestion rate (items/sec), 1 node, BigFUN 20M users (320M docs), 3 indexes, SSD:
http://cbmonitor.sc.couchbase.com/reports/html/?snapshot=oceanus_600-1460_sync_aa3f&label=6.0.0-1460-WARN&snapshot=oceanus_600-1432_sync_2116&label=6.0.0-1432-DEBUG

Unfortunately I didn't have IO stats working  when running the DEBUG tests. Its now fixed.
I'll  rerun the DEBUG test with all stats in place and update the ticket.

Logs:
172.23.96.5:kv
172.23.96.7:kv
172.23.96.8:cbas
[https://s3-us-west-2.amazonaws.com/perf-artifacts/jenkins-oceanus-766/172.23.96.5.zip
] [https://s3-us-west-2.amazonaws.com/perf-artifacts/jenkins-oceanus-766/172.23.96.7.zip
] https://s3-us-west-2.amazonaws.com/perf-artifacts/jenkins-oceanus-766/172.23.96.8.zip