I've made a 32-bit vs 64-bit speed test, using a fight sequence simulation from the game I am making. It involves multiple characters in a complex fight, and runs through lots of various code (some 4.600 lines), including running multiple AS3 scripts at run-time (some 700 lines). (https://forum.starling-framework.org/d/21339-as3-scripting-at-runtime)
I tested only the pure AS3 code simulation, with no visuals involved. After multiple tests, the average 32-bit simulation took 190ms, and the average 64-bit simulation took 200ms.
So, at least in my case, 64-bit is only slightly slower than 32-bit, which I consider negligible.
EDIT: tested on Windows 10, Intel i7-8700K