I came across this old mod

http://www.gamesas.com/topic/1321575-rel-tesv-acceleration-layer/

improves FPS by 40%

but only with v1.03 of the game. What happened to it? I can't find anything about further development of said mod

I'm not reading the whole thread, but the second or third line says,

"Big kudos to ianpatt for fixing my oversights by embedding the code as an SKSE plugin and making it available to everyone"[/size]

I would guess that's what happened to it, wouldn't you think so?

As I recall, that tool was dropped after Patch 1.4 came along because the optimizations that it was doing had been officially activated by Bethesda, along with LAA support.

Most, if not all, of the optimization performed by the tool were optimizations typically performed by compilers. To put it simply, a compiler is a program that can turn code into something a computer can understand. In other words, Bethesda, for whatever reason (Odd bugs? Lack of time to test properly?), compiled Skyrim without optimizations enabled. Due to the attention gathered by TESVAL (and SkyBoost, a tool developed by Alexander Blade, based on TESVAL), Bethesda added the optimization flags. This, in turn, made the optimizations of TESVAL/SkyBoost redundant.

If you have access to the old binaries, you can test this yourself. Measure the frames per second (FPS) at Dragon's Reach while facing Whiterun with the old binaries and then with the new ones. Far less lag with the newer binaries.

Anyhow, with the optimization flags enabled, the amount of "easy" (if I can call it that) optimizations were reduced to none (or close to it). There's still optimizations that can be made, but they're much, much more difficult to find and implement. Especially without the original source code. So, the developers Alexander Blade and TESVAL's original author (Arisu?) stopped working on their respective projects.

Also, a tool like that relies on the exact memory addresses of stuff in the executable. Each time the source code is modified and compiled, these addresses change, so the tool needs to be updated to accommodate for these changes. So, every new patch invalidates the tool. Which is why you can't use it with the current version.

P.S.: It's been a few years since then and I haven't payed much attention to the Skyrim community recently so some of the details may be slightly off, if not incorrect.