Parser rule to compare attribute data between current month load and last year Dec load

I can bet performance is poor.  Options that come to mind:

• If this is a direct connect data load, then join in the prior Dec attribute value from source so you no longer need to use a one row at a time parser rule
• Use a Transformation Rule Event handler that performs this particular transformation for all the imported rows in a single SQL statement.
• If you know the OS api well enough, you could cache a lookup table to OS Global cache.  The first time the Parser rule fires would see the lookup table does not exist, and thus create it for all passible values you could need.  For all subsequent rows, the parser rule would see the lookup table does exist in Global cache, and get the value from cache.