We perform load testing once we are done with interface
(GUI) testing. Modern system architectures are large and
complex. Whereas single user testing primarily on
functionality and user interface of a system component,
application testing focuses on performance and reliability
of an entire system. For example, a typical
application-testing scenario might depict 1000 users logging
in simultaneously to a system. This gives rise to issues
such as what is the response time of the system, does it
crash, will it go with different software applications and
platforms, can it hold so many hundreds and thousands of
users, etc. This is when we set do load and performance testing.

When to do Load testing:
Load tests are end-to-end performance tests conducted under 
anticipated production load levels. 
The primary aim of load testing is to determine response 
times of critical business processes and transactions to 
establish whether they are within Acceptable time frame or 
not.
Load testing will also measure the capability of an 
application to function correctly under load by measuring 
the systems key performance indicators (CPU, MEMORY etc). 

When to do Performance Testing:
The objectives of Performance Testing is to determine end-
to-end timing of various critical business processes and 
transactions while the system is under low load with a 
production sized database.

The best time to execute performance testing is at the 
earliest opportunity. 
Developing performance test scripts at such an early stage 
provides opportunity to identify and resolve serious 
performance problems and expectations before load testing 
commences.