Yes, I've tried that, as I said in the original post; there are no error messages.

That's a reasonable assumption, but I don't know how to diagnose the problem. Everything else runs just fine.

I would suspect that it possibly may be due to differences in our CPUs (mine is an AMD Phenom II six-core), but since it works for you on Gentoo this can't really be the case. I would begin with comparing build options on your Gentoo and Slackware system. I guess that you use march=native on Gentoo, have you tried that on Slackware?
Thinking about it, troubleshooting this issue will take some serious time due to the very long build times of webkitgtk, so I recommend to start with rebuilding the other packages first, with some luck this is not caused by webkitgtk.

@ibrunton: I noticed the flashplayer addon slowed everything down & crashed both firefox & midori. Un-installed flash and everything sped up midori most noticably. Don't know if this makes any difference on your system.

Curious, what version slackware you running? Wondering if I could run TobiSGD's packages on slackware64-13.37? I'm looking for faster browsing too. Just installed Midori SlackBuild not much faster than firefox. Wow 6core cpu... still on 2core. Will 14.1 webkit run on 13.37 or are their lib differences? Is there a luakit slackbuild?

I would just give it a try, it may be possible that it will run. Slackbuilds for luakit are available on SBo, but you will need to build webkitgtk, which takes a long time due to its inability to build parallel on multiple cores.