InnoDB tablespace critical Could the error shown on the screenshot be the reason why my site is very slow?

If so, I really need help to fix it since I am far from being an engineer!
I wouldn't think this is why your site is very slow.  You can change the alert levels by editing the following file:

vi /etc/munin/plugin-conf.d/mysql_innodb


In this file you should see:

[mysql_innodb]
env.warning 2147483648
env.critical 1073741824


If the tablespace falls below either of these levels a warning or critical is raised.  Change these limits to whatever values (in bytes) you need. If you wish to completely disable this monitoring set the values to 0:

[mysql_innodb]
env.warning 0
env.critical 0


Restart munin:

/etc/init.d/munin-node restart
su munin -c /usr/bin/munin-cron