I'm not even sure GNU-R is the most important comparison (although it is an important comparison). How does it compare to R with Intel MKL? How does it compare to other (faster) languages?
We didn't want to include comparison to R-3.5.X, because FastR itself is based on the base library of 3.4.0, but the results for GNU-R 3.5.1 almost the same as for R-3.4.0.
AFAIK ALTREP is not used that much yet inside GNU-R itself. They can now do efficient integer sequences (i.e. 1:1000 does not allocate 1000 integers unless necessary), which would save a little bit of memory in this example, but that's about it. FastR also plans to implement the ALTREP interface for packages. Internally, we've been already using things like compact sequences.