FFmpeg, x265 and slower encoding on faster PC

[Dave]

Before I head off into diags land and asking elsewhere, has anyone
seen any weirdnes with x265 encoding speeds FFmpeg on FreeBSD?

The specific weirdness isn't just slow encoding, that's expected in
comparison to, eg x264.  The specific weirdness is my 6 core
AMD Phenom II 3.3GHz based PC is encoding significantly slower
than my 2 core Intel i5 @2.6GHz (4 with hyperthreading).
 I'm wondering if either AMD is not as well supported or the 
Phemon II is just so old it doesn't have the right support. 
Encoding with X264 is far faster on the AMD the Intell box.

AMD box:
CPU: AMD Phenom(tm) II X6 1100T Processor (3311.05-MHz K8-class CPU)
real memory  = 8589934592 (8192 MB)
nvidia0: <GeForce 8400GS> on vgapci0
uname -a
FreeBSD amd.asgard.uk 10.3-RELEASE-p20 FreeBSD 10.3-RELEASE-p20 #0: Wed Jul 12 03:13:07 UTC 2017     root at amd64-builder.daemonology.net:/usr/obj/usr/src/sys/GENERIC  amd64

Intel Box
CPU: Intel(R) Core(TM) i5 CPU         750  @ 2.67GHz (2675.02-MHz K8-class CPU)
real memory  = 4294967296 (4096 MB)
nvidia0: <GeForce 9500 GT> on vgapci0
uname -a
FreeBSD testbox.asgard.uk 10.3-RELEASE-p20 FreeBSD 10.3-RELEASE-p20 #0: Wed Jul 12 03:13:07 UTC 2017     root at amd64-builder.daemonology.net:/usr/obj/usr/src/sys/GENERIC  amd64

ffmpeg version 3.3.4 Copyright (c) 2000-2017 the FFmpeg developers
  built with FreeBSD clang version 3.4.1 (tags/RELEASE_34/dot1-final 208032) 20140512
Built with default options.

I've tried swapping the video cards over to see if FFMPeG or the x265 library was using h/w acceleration.