I currently work as a sysadmin for a web agency.
My day to day work includes configuring new servers and launching the site's we've designed & developed on them.
I also regularly upgrade software, optimise configurations, fix bugs in the sites caused by deprecated features etc. etc.
I'm well practised in dealing with PHP5 issues. In January, the company I work for gained 4 clients from another agency. Their sites were made between 2010-2012. The new server's we put their sites on came with PHP5. The main bulk of work in the migration was troubleshooting PHP incompatibilities.
My plan of action for this will be to stick some monitoring tools on the server, identify the stress points, their sources and determine a pragmatic solution to them.
Then I will implement these.
I will also look at other areas where speed could be increased;
like through more thorough, aggressive caching,
database config optimisations to make the most of your server's resources,
checking if Apache is using mod_deflate to compress CSS & Javascript files before sending to the users,
optimising media files' sizes,
and, time permitting, more.