Steel Bank Common Lisp

My program is being brought to its knees by the (incredibly slow)
printing of these compiler warnings at runtime.  I'm not sure why the
compiler is running at runtime, I certainly am not calling it
directly.  I would like help, any stupid hack, to go to the debugger
and/or give some sort of contextual clue what source code I've written
is responsible.

Running grep, I think I've found the source of the warning, but I
don't grok what it means:
;;;; sbcl-0.8.17/src/compiler/aliencomp.lisp
(deftransform %sap-alien ((sap type) * * :important t)
  (give-up-ir1-transform
   ;; FIXME: The hardcoded newline here causes more-than-usually
   ;; screwed-up formatting of the optimization note output.
   "could not optimize away %SAP-ALIEN: forced to do runtime ~@
    allocation of alien-value structure"))


; in: LAMBDA NIL
;     (SB-ALIEN-INTERNALS:NATURALIZE
;    (SB-SYS:SAP+ SB-ALIEN::SAP (/ SB-ALIEN::OFFSET SB-VM:N-BYTE-BITS))
;    '#<SB-ALIEN-INTERNALS:ALIEN-RECORD-TYPE (STRUCT SDL:SURFACE
;                                                    (SDL::FLAGS #)
;                                                    (FORMAT #)
;                                                    (SDL::W #)
;                                                    (SDL::H #)
;                                                    (SDL::PITCH #)
;                                                    (SDL::PIXELS #)
;                                                    (SDL::OFFSET #)
;                                                    (SDL::HWDATA #)
;                                                    ...)>)
; ==>
;   (SB-ALIEN-INTERNALS:%SAP-ALIEN ALIEN
;                                 
'#<SB-ALIEN-INTERNALS:ALIEN-RECORD-TYPE (STRUCT
;                                                                     
     SDL:SURFACE
;                                                                     
     (SDL::FLAGS
;                                                                            #)
;                                                                     
     (FORMAT
;                                                                            #)
;                                                                     
     (SDL::W
;                                                                            #)
;                                                                     
     (SDL::H
;                                                                            #)
;                                                                     
     (SDL::PITCH ..))>)
; 
; note: unable to
;   optimize
; because:
;   could not optimize away %SAP-ALIEN: forced to do runtime 
; allocation of alien-value structure