How to deal with large amount of data in API - php

Question

I am developing a dashboard API for doing some operations related to users (create user, update, transfer money,...) by using PHP CURL to retrieve the data from an external source and display it to the front-end.
Everything was working well until the data started to grow at the database.
My senario is that each Admin after logging in to the dashboard should only do operations on his related users, that's why it takes too much time to retrieve all users from the source and then check the condition for related users and display them to this admin.
Is there a better solution for such kind of operations ?
I tried the following code to retrieve the data from the source and looped over requests since it's limited to 100 at each request.
function get_users($session_id){
$curl = curl_init();
curl_setopt_array($curl, array(
CURLOPT_URL => "THE_URL",
CURLOPT_RETURNTRANSFER => true,
CURLOPT_ENCODING => "",
CURLOPT_MAXREDIRS => 10,
CURLOPT_TIMEOUT => 0,
CURLOPT_FOLLOWLOCATION => true,
CURLOPT_HTTP_VERSION => CURL_HTTP_VERSION_1_1,
CURLOPT_CUSTOMREQUEST => "POST",
CURLOPT_POSTFIELDS =>"{\"offset\": 0,\"limit\": 100}",
CURLOPT_HTTPHEADER => array(
"x-api-token: TOKEN",
"Content-Type: application/json",
),
));
$response = curl_exec($curl);
$data = json_decode($response, true);
curl_close($curl);
$allcollection = [];
foreach ($data['users'] as $key => $value) {
if(isset($value['admin']['id'])){
if($value['admin']['id'] == $session_id){
array_push($allcollection,$value);
}
}}
$totalrows = $data['nrOfResults'];
$offset = 100;
while($totalrows >= 100){
$newcurl = curl_init();
curl_setopt_array($newcurl, array(
CURLOPT_URL => "THE_URL",
CURLOPT_RETURNTRANSFER => true,
CURLOPT_ENCODING => "",
CURLOPT_MAXREDIRS => 10,
CURLOPT_TIMEOUT => 0,
CURLOPT_FOLLOWLOCATION => true,
CURLOPT_HTTP_VERSION => CURL_HTTP_VERSION_1_1,
CURLOPT_CUSTOMREQUEST => "POST",
CURLOPT_POSTFIELDS =>"{\"offset\": ".$offset.",\"limit\": 100}",
CURLOPT_HTTPHEADER => array(
"x-api-token: token",
"Content-Type: application/json",
),
));
$newresponse = curl_exec($newcurl);
$newdata = json_decode($newresponse, true);
curl_close($newcurl);
foreach ($newdata['users'] as $key => $value) {
if(isset($value['admin']['id'])){
if($value['admin']['id'] == $session_id){
array_push($allcollection,$value);
}
}}
$totalrows = $totalrows - 100;
$offset = $offset + 100;
}
return $allcollection; }