1. The big performance issues early on were compilation speed -- Racket had a simple JIT compiler that ran almost instantly. 2. Some rewritten parts of the sys

1. The big performance issues early on were compilation speed -- Racket had a simple JIT compiler that ran almost instantly. 2. Some rewritten parts of the system, such as IO, were initially slower, but have now been improved to catch up to the previous implementation. 3. Chez Scheme didn't support some performance features that Racket BC had, such as unboxed floating point arithmetic, parallel GC, or continuation marks. Those have been added to (Racket's variant of) Chez Scheme. 4. Racket's prior compiler was somewhat more competitive with Chez Scheme than we expected, particularly for larger and more Racket-idiomatic programs.