None of the hybrid drive's I've tested myself or seen reviewed have made much of a difference in performance.
(In the worst case, it was a 5400 RPM drive and underperformed a 7200RPM HDD-only.)

+1.

Someone has already stated that they use the 2nd bay for a 2nd drive, I also do this. I replaced the DVD drive with a drive caddy with a 2ns SSD in the units , now I have 2TB of SSD in my machine and this configuration works great.
Another person mentioned that replacing the HDD on a machine is straight forward this is very true of the T520 just a few screws and you are done, probably do not even need the hardware manual. Upping the memory is not as straight forward since 2 of the DIMMS' are on the back (easy) and 2 are on the front (not as easy since the keyboard will need to be removed), if you are going to do this follow the instructions in the hardware maintenance manual. Now when I compare the process to what it takes now on a T450 or T550 it is not as simple. On both of these machines you must remove the entire back cover to access the hard drive. Many more screws and then you need to separate the 2 halves and they actually need to be pulled apart.
And still another person mentioned about the cloning process if you buy a SSD from Samsung it comes with cloning hardware and it works, but it will only copy the windows partition not any of the Lenovo partitions. I have not viewed this as a issue but you may.
Now for that last thing I think a T520 is still a great machine to use today otherwise I would not be using it as my primary development machine. Of you are running windows it just may be time for a clean install of windows to help it along. One thing I have noticed is over time the windows system files can get out of hand (I am speaking about the registry) and cause issues including performance. If I were upgrading to a SSD this would be a good time for a clean installation, just my opinion.
So why do I have a new T450s for travel this is a weight issue this machine weighs about the same as the T520 charging brick alone.
And on more thing Hybrid drives are not worth it also in my opinion and BTW I work in the computer storage industry.

Sounds like IBM's influence is waning: first the unfortunate chiclet keyboard, now service-resistant hardware. One of the best features of the ThinkPads has been their excellent serviceability. Every screw is labeled; the manuals document each step of every procedure; all you need is a minimum of confidence and a screwdriver.

Thanks for the tips. I've been pondering a Windows reinstall. My software environment is so complicated that I resist that just as much as a computer replacement, though in the end it's often not as bad as I feared.

Right now I'm leaning towards replacing the HD and doubling RAM to 16GB. If the machine still runs slowly, a clean Windows install and machine rebuild will be easier if I've got a cloned drive handy.

Unfortunately, "all but promised" seems a bit optimistic.

The relevant bit starts at 14:30. What he said at about 16:00 was, "I would love to release something like this ... for the 25th anniversary." And it seemed pretty clear from the context that by "release" he meant "announce that we've decided to produce and sell", and maybe even have a prototype.

I hope I'm being pessimistic. I used Thinkpads at work for somewhere between 15 and 20 years, and I hate the removal of 3 mouse buttons and other keyboard changes in the more recent models.

Regarding your complex software environment: Have you considered virtualization? I started using VMWare for my development and test environments roughly 12 years ago; I can be up and running on a new laptop in as much time as it takes to install VMWare and copy over the images.

I keep all of my documentation on Google Drive and use the GDrive sync service to keep copies locally. The only thing I really need to install in the host OS other than that is MS Office.

I just use Hyper V ever since they added it to Windows 8 client (and so subsequently Windows 10) since it's "free"

I had tried switching to VirtualPC a while back and, while it was OK for Windows clients, it didn't work for my various Linux images. I have a dozen or so VMs (Win2K, XP, Win7, Win10, Ubuntu, CentOS, RHEL7) that seem to function well within the VMWare Workstation Pro framework with very little maintenance so I've stuck with it.

Someone that requires only Windows clients would likely be served just as well with Hyper V, though.

Yes, that is the case for me.

I am in this exact boat. The truth is, unless you go for a workstation-class replacement (P50) it isn't worth it. The i7-2720qm in my T520 is still a fairly capable quad-core+HT processor. I've heard that though specs say 8gb RAM max, 16gb isn't a problem.

I used an X1 carbon 2nd gen for a bit. It had a nice hi-res screen (WQHD vs FHD on my T520) but was slower, had a MUCH worse keyboard, and a MUCH MUCH worse trackpoint. Sold it on ebay and went back to the T520. None of the mainstream dual-core ultrabook cpu's are a worthwhile switch.

I will say this: the complaint about the keyboard isn't what I thought it would be, though. Typing dynamics were actually decent on IBM's version of the island style keyboard. Much better than most. No - the problem was with odd key swapped placement and the missing function keys. On a better system where they didn't screw that part up I think their particular island keyboard would be just fine for my programming and command prompt filled lifestyle.

I have the third gen carbon. Love the keyboard. Weened off of a T430s

Thanks for this. Much as I resist changes, I've been hopeful I could adapt to a decent chiclet-style keyboard. Thanks for the warning about the screwed-up layout.

I've ordered an upgrade to 16GB and am shopping for an SSD. I'll probably also bite the bullet and reinstall Windows, something I approach with far more trepidation than it merits.

Who knows, maybe my eventual replacement will be a revived old-school ThinkPad.

Indeed it's not. Sandy bridge machines all take 16GB if they have 2 DIMM sockets (or 32 if they have 4) unless the manufacturer deliberately subverts it; neither Dell or Lenovo did, even on their consumer machines.

The X1 Carbon 2 did some very weird things to input, even by ultrabook standards. The key feel was above-par for an ultrabook, but the lack of real function keys, other weirdnesses with the layout, and using clickpad buttons even for the trackpoint was too much.

And coming from a quad-core CPU, it's definitely not a possible switch; heck with the Haswell generation it wasn't a worthwhile switch from the regular-power Ivy Bridge dual cores (although on Skylake it is finally caught up.)

I find the island layout tolerable on my M3800. Ditto the lack of physical page up/page down and home/end keys.

Whoa.

WHOA.

Beep beep beep.

Let's back that up.

LACK of physical page up / page down, and home / end keys.

No. Never. Ever. Nooooooo.

I can't even say how many times I press control+whattheheckthatkeyisn'tthereanymore?

Not a viable option. Just as bad as not having a trackpoint.

De gustibus, and all that. I'd rather have a full keyboard, and the trackpoint/pointint stick (which unlike the diagonal keys will be back on my next system, whether Lenovo or Dell) but neither of them are killers for me.

I miss having the physical pgup/pgdn buttons on the shoulders of the directional control, but I never particularly liked the upper-right corner home/end (or for all four on the older ones), and actually find fn-left/fn-right an improvement for those when I go back to the older machines.