Sorry just some way, but would require lot more time to write a complete documentation
1) Process Optimizations
- check Joomla extensions not used and unpublish them, very important about Joomla plugin system
2) Data query Optimizations
- check all your SEBLOD list/search and form
- on SEBLOD search/list check each match (permissive is taken ressources) and change in by exactly the good one (example on a simple select the match must be exact)
...
3) HTML/CSS optimization
- make your code clean, don't load on all pages a JS library used only on 1 page
- code reduction, compression...
4) PHP7 / PHP cache
5) Application cache system -> we never used them on Octopoos, we want only real time web site, if you make a good job on points above you site would have nice performance.
...
We managed sites with 10000 visits per day and more than 2 millions of records in database.