huge cursor lag, 2017 Macbook Air, latest version of R, not R Studio

Hi,

I'm running High Sierra on a new "2017" Macbook Air.  I upgraded R from a
previous version (3.2 or 3.3 ?) and now there is a huge lag on the curser
in the R command window itself and in script files. I am not using RStudio.

There is a page on GitHub detailing the same problem currently when using
Rstudio. I have no idea whether this problem is related or not.

Any ideas/comments?

Cheers

Kevin