If you search by entering an unqualified string in the search bar eg "Dickens", then try removing
comments from
Preferences->Searching->Columns that non-prefixed searches are limited to:
I had Calibre on an Acer netbook with Mint for a while, that made a big difference.
You might also want to try these settings
In Preferences->Miscellaneous set Max. simultaneous... jobs to 1
uncheck the Limit the max. simultaneous... setting
in Preferences->Behavior set Job Priority to Low
I know they seem counter-intuitive, but my perception is that Calibre runs faster overall with those settings, you can always reset them if they make no difference or worse
BR