So, now that I finally have a free few minutes Im trying to catch up on missed messages.
As far as creating a custom build: when you build openwrt from source, you have the opportunity to specify what exactly to include via a terminal-based customization menu. make menuconfig lets you customize userspace stuff (e.g., what packages to install from the openwrt repos). make kernel_menuconfig lets you customize kernel parameters (but can be tricky to use).
My custom firmware repo on github has the build script I used to make the firmware. Its fairly well-commented and works as a good, robust way to generate a custom firmware image with custom kernel. It also provides a fairly robust way of incorporating existing configs / kernel configs so that you dont have to reconfigure everything from scratch every time.
Note there are a lot of options you can configure...between standard config and kernel config there are probably 10,000 individual configuration "on/off" switches that you can select from. Be prepared to spend many many hours looking through these menus. SOOO many hours. Also be prepared to do a lot of "trail and error recompiling"...you all see the final polished result, But it probably took 20+ re-builds that iteratively improved each time (well, most times) to get there.
Note: the 100 hz --> 1000 hz change for the interrupt timer is one of the many options that you will be presented in the make kernel_menuconfig menu.
Regarding NSS - you are correct that (some) mediatek chips have a "network accelerator" on chip that performs a similar function to the NSS processor on ipq807x chips. BUT, @slh is correct that it is not actually the proprietary qualcomm NSS...it serves the same purpose, but is its own distinct chip. Unfortunately, this means that the ipq807x NSS work that has been done by people like @qosmio wont help you any more than your car keys help you open the door to your house. To continue the analogy: this situation is a bit like how even though your car keys and house keys are both keys that unlock a lock, that similarity is really not useful in practice if you are locked out of your house.
this thread has archived much of the process of getting NSS figured out. Its....not a trivial process to say the least, and that with open source code. I cant imagine it happening at all if mediatek is closed source...