MongoDB slow - memory usage very high - mongodb

Question

I have a 5 note replicaset mongoDB - 1 primary, 3 secondaries and 1 arbiter.
I am using mong version 4.2.3
Sizes:
“dataSize” : 688.4161271536723,
“indexes” : 177,
“indexSize” : 108.41889953613281
My Primary is very slow - each command from the shell takes a long time to return.
Memory usage seems very high, and looks like mongodb is consuming more than 50% of the RAM:
# free -lh
total used free shared buff/cache available
Mem: 188Gi 187Gi 473Mi 56Mi 740Mi 868Mi
Low: 188Gi 188Gi 473Mi
High: 0B 0B 0B
Swap: 191Gi 117Gi 74Gi
------------------------------------------------------------------
Top Memory Consuming Process Using ps command
------------------------------------------------------------------
PID PPID %MEM %CPU CMD
311 49145 97.8 498 mongod --config /etc/mongod.conf
23818 23801 0.0 3.8 /bin/prometheus --config.file=/etc/prometheus/prometheus.yml
23162 23145 0.0 8.4 /usr/bin/cadvisor -logtostderr
25796 25793 0.0 0.4 postgres: checkpointer
23501 23484 0.0 1.0 /postgres_exporter
24490 24473 0.0 0.1 grafana-server --homepath=/usr/share/grafana --config=/etc/grafana/grafana.ini --packaging=docker cfg:default.log.mode=console
top:
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
311 systemd+ 20 0 313.9g 184.6g 2432 S 151.7 97.9 26229:09 mongod
23818 nobody 20 0 11.3g 150084 17988 S 20.7 0.1 8523:47 prometheus
23162 root 20 0 12.7g 93948 5964 S 65.5 0.0 18702:22 cadvisor
serverStatus memeory shows this:
octopusrs0:PRIMARY> db.serverStatus().mem
{
"bits" : 64,
"resident" : 189097,
"virtual" : 321404,
"supported" : true
}
octopusrs0:PRIMARY> db.serverStatus().tcmalloc.tcmalloc.formattedString
------------------------------------------------
MALLOC: 218206510816 (208097.9 MiB) Bytes in use by application
MALLOC: + 96926863360 (92436.7 MiB) Bytes in page heap freelist
MALLOC: + 3944588576 ( 3761.9 MiB) Bytes in central cache freelist
MALLOC: + 134144 ( 0.1 MiB) Bytes in transfer cache freelist
MALLOC: + 713330688 ( 680.3 MiB) Bytes in thread cache freelists
MALLOC: + 1200750592 ( 1145.1 MiB) Bytes in malloc metadata
MALLOC: ------------
MALLOC: = 320992178176 (306122.0 MiB) Actual memory used (physical + swap)
MALLOC: + 13979086848 (13331.5 MiB) Bytes released to OS (aka unmapped)
MALLOC: ------------
MALLOC: = 334971265024 (319453.5 MiB) Virtual address space used
MALLOC:
MALLOC: 9420092 Spans in use
MALLOC: 234 Thread heaps in use
MALLOC: 4096 Tcmalloc page size
------------------------------------------------
Call ReleaseFreeMemory() to release freelist memory to the OS (via madvise()).
Bytes released to the OS take up virtual address space but no physical memory.
How can I detrmine what is causing this high memory consumption, and what can I do to return to nornal memory consumption?
Thanks,
Tamar