phpvms/app/Http/Middleware/MeasureExecutionTime.php

<?php
/**
 * https://stackoverflow.com/a/34894933
 */

namespace App\Http\Middleware;

use App\Contracts\Middleware;
use Closure;
use Illuminate\Http\Request;

class MeasureExecutionTime implements Middleware
{
    public function handle(Request $request, Closure $next)
    {
        // Get the response
        $response = $next($request);
        if (!\defined('LUMEN_START')) {
            return $response;
        }

        // Calculate execution time
        $executionTime = microtime(true) - LUMEN_START;

        // I assume you're using valid json in your responses
        // Then I manipulate them below
        $content = json_decode($response->getContent(), true) + [
            'execution_time' => $executionTime,
        ];

        // Change the content of your response
        $response->setData($content);

        // Return the response
        return $response;
    }
}