What is best options for simple but very low latency calculations?
I think that your requirements are unreasonable and unnecessary.  Let's go through some quick calculations to check this out.

One of the quickest, commonly available bullet is the .223 Remington, which travels about 3,000 feet/second.  In 10 ns, that bullet will have traveled 0.00036 inches.  

Now, your question was about low-latency calculations and not about actually sensing the speed of the impact.  So I am going to ignore the whole sensing problem and assume that you already know a way to sense the impact accurately and quickly enough.

With a commonly available, inexpensive FPGA running at 100 MHz I estimate that you can do the calculations in about 10 clock cycles.  10 ns per clock.  This is just a guess since you didn't say exactly what the calculations are!   Even so, this is 10x longer than your 10 ns requirement.  However, in those 100 ns the bullet has only traveled 0.036 inches!

So, even if your 10 ns requirement is real, it can be compensated for simply by moving your sensor 0.036 inches farther out!  

I chose the .223 bullet as an arbitrary example.  There are faster bullets but not significantly faster.  There are some rounds that go about 4,000 feet/second, for example.  The faster speed of these does not change the basic conclusions.

I dare say that if you are working on a personal project with projectiles that are not bullets but going faster than 4,000 feet/second then you might want re-evaluate your life choices.  If your projectiles are going slower than that, then your 10 ns requirement is not realistic and a simple mechanical rearrangement of things could solve the problem.