after months of lurking I think I just registered to reply to this thread.
Please note: I'm a *nix guy. I code. I dwelve in kernel land. I assess systems' security for a living. I think, though, this doesn't mean a thing in this thread. Cost should be a consideration.
I use Linux and OS X. My usual system while going mobile is a MBP with various VMs with Linux and Windows inside. I've been using Lightroom on OS X since three years and Bibble Pro (now Corel Aftershot Pro) on Linux for almost four. So the first real answer to the relevant OP question is: Corel Aftershot Pro. It's available for Linux now, has got commercial support, it's fast and it works. Is it like Lightroom? Conceptually yes. Is it the same? Hell, no. Better or worse? As a streamlined workflow app, probably not, but that's the case with almost all of Linux desktop apps. You just have to fiddle around, and if you've got no system administration skills whatsoever, it can be a pain sometimes, just to understand why the printer stopped working. In this case, migrating from OS X to Linux could be a thorn in your side.
Anyway, I second the advice of trying with RAM and SSD, first. Please note that 10-14 secs for a single RAW file doesn't quite sound right for RAM and storage subsystems. There is no evidence of a "old" CPU sub-system, unless something it's faulty in your system.
About moving to a wintel do-it-yourself box: it's fun and entertaining. Is it cheaper? I wouldn't say so. Whenever I go to make MY system with the latest and fastest and greatest techs in CPU, board, storage, networking and GPU I really come close to the same iMac, if I choose same quantities for everything and I do NOT get a comprehensive commercial support for the system as a whole. Moreover, for my photo hobby, I need RAM and storage, not the latest CPU or GPU unless I do a LOT of retouching with photoshop-likes apps.
People often say the macs are "closed" concerning HW and use older techs. I do NOT think so. Most often, last macs use AT MOST 2nd-3rd generation in every sub-system: CPUs, chipsets, storage, GPUs. What is the real problem? They are crippled by marketing: why on earth can't I get a notebook with 64GBs of RAM? So to keep battery stats real? So to force me to buy the coffee can? Probably.
But I just don't see why a 15'' mbp with 16GB RAM and SSD can't let you fly opening your raws. There is no such a system without flaws and that costs really less then others when specs are maxed out with latest technologies. The problem with OS X and Windows, though, is that filesystem/registry cleaning is not transparent to the user and RAM usage is sometimes, well, stupid. You should get really better with Linux if you knew what to do and what to look for, but only after configuring every other thing that you get for granted and already working out of the box with Win and OS X. So choose carefully.
Going to external storage: please buy/use USB3 / thunderbolt / e-sata / fw800 / usb2 in this order (cost-wise) and use, if possible, a single bus for your discs. Do not plug your external storage on the same bus shared by other devices: this is most true with USB2 and FW.
Last problem with macs: the great hw/sw integration, very closed and very tuned, though, to user satisfaction, means ANY kind of quirks whenever you're beginning to experience HW faults in the logic board, in some chipset, in the RAM, in the CPU itself. BUT you should see problems in many more contexts, not only while opening RAWs in Lightroom.
Oh, I shoot Canon, obviously