Description : The process of page rendering has been palallelized, whitch leads to a better response time, especially in the case when page waits for some external services to get content.
This logic can be turned on/off in the following section of "/App_Data/Composite/Composite.config" file:
.....
 <Composite.Parallelization.Plugins.ParallelizationProviderConfiguration>
 <Parallelization enabled="true">
   .....
   <add name="XsltBasedFunction. Parameters evaluation" enabled="true" />
   <add name="XsltBasedFunction. Function calls evaluation" enabled="true" />
   <add name="Functions. Executing nested function calls" enabled="true" />
   <add name="PageRenderer. Embedded function execution" enabled="true" />
  </Parallelization>
 </Composite.Parallelization.Plugins.ParallelizationProviderConfiguration>

Note: It is necessary to avoid changing  System.Web.HttpContext.Items collection while rednering "in parallels", since it may lead to a race condition.
Exactly for that reason sites, which are upgraded from C1 1.2, have this logic disabled by default.