Hi there
Please search it on forums. There are many topic about it already 🙂
# Check and improve queries
It’s development work. Need check and know what’s needed, what’s not needed for specific cases. Than improve it.
# Reduce site size ( images / css / html / etc.. )
It’s frontend work. Reduce css selector will your browser read and parse faster. Also reduce site size for loading.
You will also can do CDN for these
# Improve server configuration
It’s sysadmin job 🙂
In finally except you have much budget and target for very large site than you will need to do that. Or … just leave it as it’s.
Just my though.
Thank you,
Viet Vu