Could it be that the hint that vr is looking for is simply to "define PERL_NO_GET_CONTEXT" ?

"Simply"!? No, it isn't simple :-) Not for me. And yes, with this define, a no-op stub performs equally fast both in Linux and threaded Win32, and time for test in OP is 3.7 sec, while it was ~5 and ~11, respectively. (So, BrowserUk, it looks like this stub wasn't optimized away.) Thank you for link and explanation, now at least I have some idea what's going on. The Hash::Util has this magic incantation as first line of its XS, while Array::RefElem hasn't anywhere, so it explains their different speed, too. My real C code calls SvPV and others, with this define it stops working as explained in link you provided, I'll have to solve this, but, these are details to work out.