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18 hours ago, ptityop said:

So we set up a website a few months ago, sped was optimum but with time we notice a dramatic decrease in speed ... We have followed all guidances regarding cache usage , we kept add ons to a strict minimum ... and now we see that the speed is not as it was before. What can make an install decrease in performance when no changes have been made since the production launch ? Version is 

1.7.8.8

Thank you in advance for your insights!

If optimum and no change then one would suspect a mysql table with many rows.  Generally this can be measured reasonably well using https://www.webpagetest.org/ to determine TTFB, time to first byte, if 2+ seconds, much of that time can usually be waiting on data. 

The first thing I would do is enable mysql slow query log, here are Plesk instructions your control panel may differ.  Set to two seconds and test the shop and see what if any mysql calls are taking over two seconds.  or one second.

also note:  mysql on 'all' but higher end fully managed, and then not, my.cnf has default settings.  You can learn what to tune via suggesting from phpmyadmin.

when I do performance work, I first tune the hosting...it's more straightforward process compared to finding some needle in the PS haystack. 

hosting is like a fish tank, you always wished you bought a bigger one.  with cloud solution it's much easier to upgrade power than on legacy..

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tips: 

avoid any module with the word speed or cache in it.  PS provides non static dynamic pages, any variable stored in some cache can create unpredictable and non reproducible customer reported issues.

I recommend https://prestaspeed.com/ they are fanatics about performance, I think they provide the service just so they can show off.

I do some performance dev, our new css/js pro...we da bomb....https://prestaheroes.com/collections/performance