Today's problems have been mostly caused by the huge traffic drived by the latest post on our blog
For all the day the load average on the server has been above 200, and our 8 processors never stop to run.
That permitted to put in evidence large optimization margins still existing on this server: discrete performance tuning has been constantly in place in last two months, targetting above all the forum, but other major portions of the whole Arduino web platform still needs some revision.
lardconcepts:
In addition you could switch from Apache to something fast like Nginx, and the version of php is hopelessly out of date.
nginx has been considered but actually is not an option: migration would require a large porting of current Rewrite rules, now massively used to glue together all the pieces. Not impossible, but requires some time.
The very first and easiest free thing to do would be to use Cloudflare
I've observed bad performance of Cloudflare on at least one website far smaller than the Arduino's one, it impressed me quite negatively.
Did you had some positive experience?
Then you could add in Varnish cache
Yes, this is already on my todolist 
Sorry for the bad service seen today, your sysadmin is working for you.