1. Open httpd.conf, enable this line:

LoadModule ratelimit_module modules/mod_ratelimit.so

2. Find DocumentRoot block and add settings like this:

DocumentRoot "D:/Xampp/htdocs"
<Directory "D:/Xampp/htdocs">
    ...
    SetOutputFilter RATE_LIMIT
    SetEnv rate-limit 10 # It means 10 KiB/s
    ...
</Directory>

3. Restart Apache

Reference: https://youtu.be/lv-8YADwntk