MongoDB - CPU utilisation goes beyond 70% on huge update - mongodb

Question

I have a mongodb collection test. An update operation is done on this collection every 30 seconds. Sometimes the cpu utilization goes above 80% and connection times out. Also when I enabled profiling and checked /var/log/mongodb/mongod.log, the time taken for the update operation to complete is more than 40 seconds. Update query is very big and it looks like this.
Update Query
test.update({_id: xxxxxxxx},{ $set: {
c_id: "5803a892b6646ad17b2b7a67", s_id: "58f29ee1d6ee0610543f152e",
c_id: "58f29a38c91379637619605c", m: "78a3512ad29c",
date: new Date(1505520000000), l.0.h.0.t: 0, l.0.m.0.0.t: 0,
l.0.h.0.t: 0, l.0.m.0.0.t: 0, l.0.h.0.r: 0, l.0.m.0.0.r: 0,
l.0.h.0.r: 0, l.0.m.0.0.r: 0, l.1.h.0.t: 0, l.1.m.0.0.t: 0,
l.1.h.0.t: 0, l.1.m.0.0.t: 0, l.1.h.0.r: 0, l.1.m.0.0.r: 0,
l.1.h.0.r: 0, l.1.m.0.0.r: 0, l.0.m.0.1.t: 0, l.0.m.0.1.t: 0,
l.0.m.0.1.rxb: 0, l.0.m.0.1.r: 0,
l.1.m.0.1.t: 0, l.1.m.0.1.t: 0, l.1.m.0.1.r: 0, l.1.m.0.1.r: 0,
l.0.m.0.2.t: 0,
.....................
.....................
.....................
.....................
.....................
}})
The query is very large, I have only posted a part of the query and the schema is also very much involved. How can we improve the performance of this update query? Should I optimize schema to reduce the number of nested documents?
I would like to know what are steps I can take to improve this update query.
.explain() output for Update Query
{
"queryPlanner" : {
"plannerVersion" : 1,
"namespace" : "test.stats",
"indexFilterSet" : false,
"winningPlan" : {
"stage" : "UPDATE",
"inputStage" : {
"stage" : "IDHACK"
}
},
"rejectedPlans" : []
},
"serverInfo" : {
"host" : "abcxyz",
"port" : 27017,
"version" : "3.4.4",
"gitVersion" : "988390515874a9debd1b6c5d36559ca86b4babj"
},
"ok" : 1.0
}