What is the performance bottleneck when launching large applications on OS X?
Activity Monitor is a good starting point. At a minimum, you can determine whether your system is becoming CPU-bound when you start the large application. It can also show how system memory is being utilized and whether excessive swapping is taking place. An Apple KB notes:


  Tip: Page outs occur when the Mac has to write information from RAM to
  the hard drive (because RAM is full).  Adding more RAM may reduce page
  outs.


This Quora post has a reference to another good tool - the Shark utility from the Apple Developer Tool Suite:


  Shark from the Developer Tools suite (CHUD) provide even more powerful
  process inspection capabilities and organizes bottom-up and top-down
  trees of where your time goes for the entire space of function calls,
  and is even decent at explaining where the bottlenecks are, sometimes
  suggesting low-level optimizations.


Apple's Developer Library notes:


  Shark is an advanced tool for sampling or tracing a single application
  or all running applications. Like Instruments, Shark provides a set of
  powerful analysis tools with an easy-to-use interface to help you
  gather performance data. Shark helps you isolate problems quickly by
  providing a rich set of data-mining features and is an indispensable
  tool for finding performance bottlenecks. It provides optimization
  tips and help with analyzing assembly-level operations. Because it
  traces operations down at the kernel-level of the system, Shark can
  also provide you with tremendous insight about the behavior of your
  application relative to the rest of the system.
  
  Among the types of operations you can perform in Shark are the
  following:
  
  
  Statistical sampling of your application over a period of time 
  System-level tracing 
  Malloc tracing 
  Static analysis 
  L2 Cache profiling 
  Processor bandwidth analysis 
  Java code analysis 
  
  
  In addition to basic sampling, Shark also offers the windowed time
  facility feature for several of its sampling options. The windowed
  time facility tells Shark to record and process a finite buffer of the
  most recently acquired samples. This feature lets Shark record data
  continuously and for long periods of time in the background. You can
  then direct Shark to stop sampling only after something interesting
  occurs in your code and to display the samples related to that
  interesting behavior. Thus, you can use this feature to determine what
  part of your program’s execution is interesting after it occurs.
  
  For a quick example of how to use Shark, see “Using Shark.” For
  detailed information about Shark, see Shark User Guide.


The page also lists a number of other tools in the developer tool suite that can be used to monitor your system's performance and hopefully pin down any potential bottlenecks before you buy the second SSD.