Slow SUBDOC_MULTI_MUTATION operation

Couchbase Server
1

HI,
I’m using Couchbase Community Edition 5.0.1 build 5003. My system uses sub-documents operations. Some of update requests fail with timeout. I checked couchbase logs and I noticed that there are a lot of logs with slow SUBDOC_MULTI_MUTATION operations in babysitter.log file:

[ns_server:info,2019-05-15T11:50:52.540Z,babysitter_of_ns_1@127.0.0.1:<0.78.0>:ns_port_server:log:223]memcached<0.78.0>: 2019-05-15T11:50:52.339444Z WARNING 70: Slow SUBDOC_MULTI_MUTATION operation on connection: 549 ms ([ 10.xx.xx.xx:39264 - 10.xx.xx.xx:11210 (profile) ]) opaque:0x7c7c2b00

[ns_server:info,2019-05-15T11:50:54.135Z,babysitter_of_ns_1@127.0.0.1:<0.78.0>:ns_port_server:log:223]memcached<0.78.0>: 2019-05-15T11:50:53.934578Z WARNING 43: Slow SUBDOC_MULTI_MUTATION operation on connection: 2022 ms ([ 10.xx.xx.xx:44610 - 10.xx.xx.xx:11210 (profile) ]) opaque:0x58e26400

[ns_server:info,2019-05-15T11:50:54.488Z,babysitter_of_ns_1@127.0.0.1:<0.78.0>:ns_port_server:log:223]memcached<0.78.0>: 2019-05-15T11:50:54.287664Z WARNING 47: Slow SUBDOC_MULTI_MUTATION operation on connection: 2371 ms ([ 10.xx.xx.xx:40922 - 10.xx.xx.xx:11210 (profile) ]) opaque:0xcc623f00

[ns_server:info,2019-05-15T11:50:55.281Z,babysitter_of_ns_1@127.0.0.1:<0.78.0>:ns_port_server:log:223]memcached<0.78.0>: 2019-05-15T11:50:55.080572Z WARNING 43: Slow SUBDOC_MULTI_MUTATION operation on connection: 1144 ms ([ 10.xx.xx.xx:44610 - 10.xx.xx.xx:11210 (profile) ]) opaque:0x59e26400

[ns_server:info,2019-05-15T11:52:22.893Z,babysitter_of_ns_1@127.0.0.1:<0.78.0>:ns_port_server:log:223]memcached<0.78.0>: 2019-05-15T11:52:22.692454Z WARNING 75: Slow SUBDOC_MULTI_MUTATION operation on connection: 773 ms ([ 10.xx.xx.xx:36744 - 10.xx.xx.xx:11210 (profile) ]) opaque:0x31313800

Could you tell me how could I find the root problem of that delays? What should I check?