[R] Different performance with different R versions

[cowan robin]

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?