How to optimise MongoDB aggregation - mongodb

Question

Our MongoDB database has a collection of companies and a collection of users. I'm running an aggregation pipeline, the details of which vary based on various filters chosen by an administrator, but below is a simple example. The companies collection has approximately 15 entries, and there are currently around 40,000 entries in the users collection.
Our aggregation returns user data, but must contain the companyname from the companies collection (there may also be filtering on the company depending on what options are chosen). A user document contains an array of subdocuments for reports - we need to include the last report date for possible sorting/filtering, too. The only other non-standard thing is that there is an object field keydata which needs to be included for searching. This object will be different for every user.
With only 40,000 records in the users collection, this aggregation is currently taking approximately five seconds. Eventually, we will have millions of records in the database, so I need a far more performant way of searching records.
Any pointers, please?
[
{
$lookup: {
from: "users",
let: {
cref: "$companyref",
companyname: "$companyname",
},
as: "users",
pipeline: [
{
$match: {
$expr: {
$eq: ["$$cref", "$companyref"],
},
},
},
{
$addFields: {
lastreportdate: {
$arrayElemAt: [
{
$slice: ["$reports.date", -1],
},
0,
],
},
kd: {
$objectToArray: "$keydata",
},
name: {
$concat: ["$firstname", " ", "$lastname"],
},
companyname: "$$companyname",
},
},
{
$match: {
$or: [
{
"kd.v": /searchstring/,
},
{
name: /searchstring/,
},
{
email: /searchstring/,
},
{
userref: /searchstring/,
},
{
username: /searchstring/,
},
],
},
},
{
$project: {
kd: 0,
lastreportdate: 0,
},
},
],
},
},
{
$unwind: {
path: "$users",
preserveNullAndEmptyArrays: false,
},
},
{
$replaceRoot: {
newRoot: "$users",
},
},
{
$project: {
userref: 1,
keydata: 1,
email: 1,
zipcode: 1,
companyref: 1,
companyname: 1,
firstname: 1,
lastname: 1,
name: 1,
reports: 1,
lastlogin: 1,
},
},
]
Collection companies has an index on companyref and collection users has an index on userref.