InnoDB Tablespace critical error in great need of a fix Link to screenshot : http://www.nouvellesduquartier.com/i/1/p/Munin_%20Critical_MySql_InnoDB_.JPG

Question: Could the error shown on the screenshot be the reason why my site is very slow?
Your free space in the current innodb file (ibdata[1] ?) is decreasing.
It doesn't necessarily mean that your website is slow.

Maybe the number of users (concurrent users) has increased in the last few days ? (also causing lots of inserts int he DB?) 

There is no error in the print screen. it's just your monitoring system, reporting a value that have reached the critical threshold.