Sysctl.conf

Quote from man page:

DESCRIPTION
     sysctl.conf contains a list of sysctl(8) variable assignments that is
     read at system startup by rc(8) early on in the boot sequence.

     The file is made up of sysctl(8) variable assignments (variable=value)
     with comments designated by a hash mark (`#').

The variable list that can bet set here is pretty long, take a look at them if you wish by typing:

man -s 8 sysctl

But the ones that should mostly interest you are:

Kernel limit for fildescriptors:

• kern.maxfiles

Buffer sizes, don't set them too high for nothing either:

• net.inet.tcp.recvspace
• net.inet.udp.recvspace
• net.inet.tcp.sendspace
• net.inet.udp.sendspace

Other settings that may improve performance:

• kern.somaxconn
• net.inet.ip.maxqueue

/This is still incomplete, feel welcome to add what you learned from personnal experience here!