Likewise, I opt to use (chicken fixnum) but this is
because I discovered that SRFI-143 is so egregiously slower than
(chicken fixnum) that it was causing serious performance regressions in
many of the array APIs.
That's very odd. The Chicken-specific library wrapper for SRFI 143 at GitHub (srfi-143.scm, which needs to be updated to Chicken 5) makes use of Chicken's native fx* procedures. I don't know how the Chicken (srfi 143) egg was built, though; it may be using rubber-chicken.scm and fxcore.scm, which are slow emulations meant for complete portability.
If you have time/energy, it would be useful to make Chicken's implementation do what it was originally intended. See the "Implementation" section in the SRFI, or feel free to ask me for explanations of details.