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:
In this file you should see:
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:
InnoDB stores data in a tablespace. By default, there is one single tablespace and data of all the databases is stored in one file. This file has data dictionary, tables, as well as indexes in it. There is a global parameter innodb_data_file_path that defines this tablespace file. It has a syntax like ibdata1:256M:autoextend, this means at the beginning a file of size 256 MB will be created and then whenever the data size exceeds this, the file will be auto-extended. The innodb_autoextend_increment variable defines in MB's that by how much each increment should be.
Reference link wherein it has mentioned in depth about InnoDB tablespace which can be helpful to you : http://optimmysql.blogspot.in/2007/07/innodb-tablespace-size.html