Elastic APM PHP Agent slows down PDO calls

[vanilla-lake]

Describe the bug
PDO calls made with the Elastic APM PHP Agent extension installed are much slower than when it is not installed. In my tests, it was about three times slower with the Elastic APM PHP Agent installed.

To Reproduce
Steps to reproduce the behavior:

1. Create a new directory.
2. Save the following code to test.php inside the new directory:

<?php

declare(strict_types=1);

$pdo = new PDO(
    "pgsql:host={$_ENV["DB_HOST"]};dbname={$_ENV["DB_NAME"]}",
    $_ENV["DB_USER"],
    $_ENV["DB_PASSWORD"]
);

for ($i = 0; $i < 10000; $i++) {
    $pdo->exec("SELECT 1");
}

3. Save the following code to Dockerfile inside the directory:

FROM --platform=amd64 php:8.1-apache

RUN curl -fsSL https://github.com/elastic/apm-agent-php/releases/download/v1.13.0/apm-agent-php_1.13.0_amd64.deb > /tmp/apm-agent-php.deb \
    && dpkg -i /tmp/apm-agent-php.deb \
    && rm /tmp/apm-agent-php.deb

RUN apt-get update && apt-get install -y \
    libpq-dev \
    time \
    && rm -rf /var/lib/apt/lists/*

RUN docker-php-ext-configure pdo_pgsql \
    && docker-php-ext-install -j$(nproc) pdo_pgsql

COPY test.php /opt/

CMD ["time", "-p", "php", "/opt/test.php"]

4. Save the following code to docker-compose.yml inside the directory:

services:
  php:
    build: .
    depends_on:
      - postgres
    environment:
      DB_HOST: postgres
      DB_NAME: postgres
      DB_USER: postgres
      DB_PASSWORD: 12345

  postgres:
    environment:
      POSTGRES_PASSWORD: 12345
    image: postgres

5. Build and run the container image by running the following commands inside the directory:

docker compose build
docker compose run --rm php

6. Comment out the following lines in Dockerfile:

#RUN curl -fsSL https://github.com/elastic/apm-agent-php/releases/download/v1.13.0/apm-agent-php_1.13.0_amd64.deb > /tmp/apm-agent-php.deb \
#    && dpkg -i /tmp/apm-agent-php.deb \
#    && rm /tmp/apm-agent-php.deb

7. Build and run the container image by running the following commands inside the directory:

docker compose build
docker compose run --rm php

8. Notice that the real execution time is much longer when the Elastic APM PHP Agent is installed.

Expected behavior
The Elastic APM PHP Agent should not have such a huge performance impact.

[bmdevel]

We have this issue as well on our production servers. Replaying this example shows the huge impact of the current client implementation

with apm

docker compose run --rm php
real 0.41
user 0.25
sys 0.05

without apm

docker compose run --rm php
real 0.21
user 0.01
sys 0.07