I've installed Xdebug 3.5.0 with PHP versions 8.5, 8.4, 8.3 and all are very slow compared to Xdebug 3.4.x.

Here is test code.

<?php
// Start the benchmark
$start_time = microtime(true);

// Code block to benchmark (example: a simple loop)
$iterations = 1000000;
for ($i = 0; $i < $iterations; $i++) {
    // Perform some operation, e.g., string concatenation
    $result = "Test string" . $i;
}

// Stop the benchmark
$end_time = microtime(true);

// Calculate and display the total time taken in seconds
$total_time = round(($end_time - $start_time), 4);
echo 'Code execution time: ' . $total_time . ' seconds.';

With Xdebug 3.4.x, the result will be around 0.12xxx - 0.13xxx seconds.
But with Xdebug 3.5.0 the result go up to 9 seconds+ or more.

Tested with PHP 8.3, 8.4, 8.5 on Windows.

php -v

PHP 8.5.0 (cli) (built: Nov 18 2025 08:16:50) (NTS Visual C++ 2022 x64)
Copyright (c) The PHP Group
Zend Engine v4.5.0, Copyright (c) Zend Technologies
with Xdebug v3.5.0, Copyright (c) 2002-2025, by Derick Rethans
with Zend OPcache v8.5.0, Copyright (c), by Zend Technologies

Web server:
Apache/2.4.46 (Win64) OpenSSL/1.1.1i mod_fcgid/2.3.10-dev
Non thread safety.
Compiler: Visual C++ 2022
Architecture: x64

php.ini config:

; Xdebug
zend_extension = 'D:/wwwserver-x64/php/xdebug/php_xdebug-3.5.0-8.5-nts-vs17-x86_64.dll'
xdebug.profiler_append = 0
xdebug.start_with_request = trigger
xdebug.mode = develop,profile
xdebug.output_dir = 'D:/my-dev-programs/wwwserver-x64/tmp/xdebug'
xdebug.profiler_output_name = 'cachegrind.out.%p%H%R'

Xdebug log:

[8168] Log opened at 2026-01-14 04:42:56.268240
[8168] [Config] WARN: Can't create control Named Pipe (0xe7)
[8168] [Config] INFO: Trigger value for 'XDEBUG_TRIGGER' not found, falling back to 'XDEBUG_PROFILE'
[8168] [Config] INFO: Trigger value for 'XDEBUG_PROFILE' not found, so not activating
[8168] Log closed at 2026-01-14 04:43:10.227919