[R-SIG-Mac] Different performance with different R versions

[Cowan, R (MERIT)]

 I am running a small simulation, and getting very different run times when I use different versions of R. 
   Two set-ups using the same machine (MacBook Pro 2013 vintage)

   1. R version 3.1.3  running on system OS X 10.9.5

> system.time(source("simulationR-R.R"))

     user  system elapsed 
     3.890   0.061   3.965 

   Compared to

   2. R version 3.5.1  running on system OS X 10.12.6

> system.time(source("simulationR-R.R"))

     user  system elapsed 
   277.924   2.087 280.841 

The source code is identical. This is a pretty big difference running the same code on the same hardware.
Before submitting the code, is this a known issue?


Thanks,
Robin Cowan