Anytime I have had ThingWorx architecture concerns, I have been redirected to the PTC ThingWorx guide linked below. I do not believe you need a PTC account to view it, but if so it is free.
ThingWorx 8 High Availability Administrators Guide
http://support.ptc.com/WCMS/files/173281/en/ThingWorx_8_High_Availability_Administrators_Guide.pdf
In your case where you have big load concerns, the guide recommends using
two ThingWorx instances to handle the load.
At least two ThingWorx instances are required for HA configuration. A
single instance is started, which becomes leader and fully connects to
the database. Standby servers boot up and can become the leader if
needed, but they do not fully connect to the database or load
information like the leader does. All ThingWorx servers have a service
that is called by the load balancer, which indicates their
availability. Different codes identify the leader, which receives
traffic, and standby nodes, which do not receive traffic but may
become leader.
High-Level Architecture example from the referenced guide:

The Load Balancer determines which ThingWorx instance is to be used by the user. Usually it is used to determine which is available in a redundant architecture (which is what makes it Highly Available). However, it can also be used to determine which to use based on performance. In PTC's HA Admin Guide, they use HAProxy (see page 47) as the Load Balancer. See Section 3.2 of the HAProxy Config Doc for how to configure based on performance.
Hope this helps! It is a pretty open-ended topic