Docker - ubuntu 21.04 - low host CPU usage

I'm running a LAMP application using docker compose on Ubuntu 21.04. The application is extremely slow.

Checking the docker stats I'm seeing that the CPU % usage, while loading a page, is most of the time around 0.01*% except few spikes, that unfortunately lasts few seconds, where the CPU usage hits an higher percentage (around 20%- 30%).

Docker info:

WARNING: Error loading config file: .dockercfg: $HOME is not defined
Client:
Context:    default
Debug Mode: false

Server:
Containers: 13
 Running: 3
 Paused: 0
 Stopped: 10
Images: 34
Server Version: 19.03.13
Storage Driver: overlay2
 Backing Filesystem: extfs
 Supports d_type: true
 Native Overlay Diff: true
Logging Driver: json-file
Cgroup Driver: cgroupfs
Plugins:
 Volume: local
 Network: bridge host ipvlan macvlan null overlay
 Log: awslogs fluentd gcplogs gelf journald json-file local logentries splunk syslog
Swarm: inactive
Runtimes: runc
Default Runtime: runc
Init Binary: docker-init
containerd version: 8fba4e9a7d01810a393d5d25a3621dc101981175
runc version: 
init version: fec3683
Security Options:
 apparmor
 seccomp
  Profile: default
Kernel Version: 5.11.0-22-generic
Operating System: Ubuntu Core 18
OSType: linux
Architecture: x86_64
CPUs: 8
Total Memory: 15.36GiB
Name: fabrizio-XPS-13-9305
ID: C7F4:H3BH:6S65:I24Q:PJ4W:EEBN:SXMH:V2GN:B67C:XYUL:R56O:5AKX
Docker Root Dir: /var/snap/docker/common/var-lib-docker
Debug Mode: false
Registry: https://index.docker.io/v1/
Labels:
Experimental: false
Insecure Registries:
 127.0.0.0/8
Live Restore Enabled: false

Here is docker-compose.yml file:

version: '3'
services:
  apache:
    depends_on:
      - mariadb
    build:
      context: .
      dockerfile: apache/Dockerfile
      args:
        URL : 'fp.example.com'
    environment:
      - URL=fp.example.com
    image: apache
    ports:
      - "80:80"
      - "443:443"
    volumes:
      - ../fp.example:/var/www/fp.example
      - ./logs/apache:/var/log/apache2
    links:
      - mariadb:database"
      - solr:solr"

  mariadb:
    image: mariadb:10.5-focal
    ports:
      - "3306:3306"
    volumes:
      - ./mysql:/var/lib/mysql
    environment:
      MYSQL_ROOT_PASSWORD: ***********
      MYSQL_DATABASE: *********
      MYSQL_USER: ***********
      MYSQL_PASSWORD: ***********
  solr:
    image: solr:8
    ports:
      - "8983:8983"
    volumes:
      - ./solr:/var/solr

Here is my apache Dockerfile:

FROM ubuntu:20.04

LABEL Description="Ubuntu Lamp [APACHE PHP MYSQL]" \
    License="Apache License 2.0" \
    Usage="" \
    Version="1.0"

ARG URL

ENV TERM=xterm\
    TZ=Europe/Berlin\
    DEBIAN_FRONTEND=noninteractive

RUN apt-get update
#RUN apt-get upgrade -y

RUN apt-get install -y --fix-missing \
    tar \
    zip \
    unzip \
    sed \
    nano \
    apache2 \
    libapache2-mod-php \
    php \
    php-cli \
    php-common \
    php-dev \
    php-fpm \
    php-gd \
    php-json \
    php-mbstring \
    php-mysql \
    php-readline \
    php-soap \
    php-tidy \
    php-xdebug \
    php-xmlrpc \
    php-zip \
    composer
# -- CHERRY -- #

RUN mkdir /var/www/$URL
RUN chown -R www-data:www-data /var/www/$URL

COPY /apache/apache.conf /etc/apache2/sites-available/
COPY /apache/php.ini /etc/php/7.4/apache2/
COPY /apache/cert/myCA.crt /usr/local/share/ca-certificates/
COPY /apache/cert/crt /etc/apache2/ssl/
COPY /apache/cert/key /etc/apache2/ssl/

# -- VOLUMES -- #
VOLUME /var/www/$URL
VOLUME /var/log/apache2


#APACHE
RUN a2enmod rewrite
RUN a2enmod ssl
RUN a2dissite 000-default
RUN a2ensite apache
RUN phpenmod xdebug
RUN phpenmod tidy
RUN update-ca-certificates

#XDEBUG
RUN echo "xdebug.remote_enable = 1" >> /etc/php/7.4/mods-available/xdebug.ini
RUN echo "xdebug.remote_host = docker.for.mac.host.internal" >> /etc/php/7.4/mods-available/xdebug.ini
RUN echo "xdebug.remote_port = 10000" >> /etc/php/7.4/mods-available/xdebug.ini
RUN echo "xdebug.remote_autostart = 1" >> /etc/php/7.4/mods-available/xdebug.ini
RUN echo "xdebug.remote_log=/var/log/apache2/xdebug.log" >> /etc/php/7.4/mods-available/xdebug.ini

EXPOSE 80
EXPOSE 443

CMD service apache2 start && tail -f /dev/null

I'm posting this here since the same project, running on mac and windows, is working nicely. So i think the problem might comes from docker on ubuntu.

Thanks all in advance.