low memory used is good or not??

Today, I check the percentage of memory used in my Cpanel, i notice that memory used below 10%, it looks fine .. but
it is quite obvious that my forums(phpbb)speed slow down because of this low memory used..

normally, my memory used is 40-70%... i dont know why the memory usage dropped suddenly...

please give me advice, thanks
low memory used is good or not??

 

 

 

 

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