They took the opportunity to move some of the infrastructure from C to Racket itself:

   Mostly, we did reimplement the C stuff in Racket. The I/O
   subsystem, the concurrency subsystem (which includes the 
   scheduler for “green” threads, Concurrent ML-style events,
   and custodians), and the regexp matcher were all rewritten
   in Racket. Those pieces followed the rewrite of the macro 
   expander in Racket.

So there was a slowdown until those pieces matured. But now they get the benefit of having all those pieces in higher level of abstraction and more accessible to Racket programmers.