One can successfully force install Inline::C, but it's unusable

While that's so for the latest version of Inline::C, it's quite simple to install older versions of Inline::C on the unthreaded Windows (as they don't carry the Win32::IPC baggage that comes with recent versions).
So, I installed Inline-0.55, though I perhaps didn't need to go that far back.
Here are the results using vr's original one-liners:

On threaded perl-5.26.0 with current Inline::C version 0.78:

>perl -MTime::HiRes=time -MInline=C,"void foo(){}" -wE"$t=time;foo()fo
+r 1..1e8;say time-$t"
11.0136189460754

>perl -MTime::HiRes=time -wE"$t=time;sub foo(){}foo()for 1..1e8;say ti
+me-$t"
5.39761018753052
[download]

On threaded perl-5.26.0 with Inline::C version 0.55:

>perl -MTime::HiRes=time -MInline=C,"void foo(){}" -wE"$t=time;foo()fo
+r 1..1e8;say time-$t"
10.4052181243896

>perl -MTime::HiRes=time -wE"$t=time;sub foo(){}foo()for 1..1e8;say ti
+me-$t"
5.58481001853943
[download]

On unthreaded perl-5.26.0 with Inline::C version 0.55:

>perl -MTime::HiRes=time -MInline=C,"void foo(){}" -wE"$t=time;foo()fo
+r 1..1e8;say time-$t"
4.92960906028748

>perl -MTime::HiRes=time -wE"$t=time;sub foo(){}foo()for 1..1e8;say ti
+me-$t"
7.65961289405823
[download]

It therefore appears that reverting to an older version of Inline::C makes very little difference, whereas using Inline::C on an unthreaded Windows perl-5.26.0 markedly improves performance when calling Inline::C subs from perl.
Unfortunately, it also seems that calling perl subs on an unthreaded Windows perl-5.26.0 takes about 30% longer (as compared to the time it takes on the threaded perl).

Of course, things might be quite different on the soon-to-be-released perl-5.28.0.
And things might also be quite different on 32-bit builds of perl.

Cheers,
Rob