"We just can't be bothered" or "we don't think they're much good, and we don't have a need for them" would be a much more realistic answer.
Go isn't interested in either being slow. Any generics solution has to have fast runtime, because they need to replace the builtin parametric slices and maps. And the compiler is already too slow, in fact there's a lot of work slated to try and make it faster.
Lots of effort went into studying the problem, so it most definitely isn't an issue of "can't be bothered".
I think the error in that reasoning is attributing this issue to generics specifically when this is actually a problem with code reuse in general.
Say I have a linked list and want to use it to hold both integers or strings. My two options, in any language—whether that language has generics or not—are: (1) write specialized code for every type I want to use it with (go generate), potentially bloating the code and resulting in extra compilation time; (2) use some sort of existential type (interface{} in Go) and share the code but pay a performance cost at runtime. The dilemma arises because of the problem itself, not because of generics.
It is of course right that there is a tradeoff here (although there are many other potential solutions that aren't at one extreme or the other, .NET-style JIT compilation or intensional type analysis for example). But not having generics doesn't eliminate the tradeoff. Generics are just a way for the compiler to automate the work that a programmer would otherwise have to do. If you don't have generics, you still have that dilemma, except that you have to write the code yourself instead of the compiler doing it for you.
It seems (and this is purely an analysis of my own experience and the large amount of Go I read inside Google) that the types of data structures and algorithms I need parametrized follow a power law distribution. Mostly I need a list, or a hash map, and the builtin slice and map types almost always meet my needs. For the long tail of parameterized types in that power distribution, it appears that either it is not a performance hot spot, in which case I can borrow from the dynamic dispatch tools in the language (that is, some variant of an interface{}), or performance is so critical that even in C++ there is no obvious generic data type.
The latter is an interesting case I ran into in my prior life as a C++ programmer, and in OCaml. Someone had created a generic version of a data structure (usually more than one someone), but what I needed in my performance hot spot was some variant that wasn't captured by the parameterization.
An old example that comes to mind: I inherited a struct whose memory layout described a wire protocol. It had an 8-byte piece of padding in the middle of it. That was the perfect place to hide the pointer for a linked list I wanted to create in an intermediate step. Of course, none of the many template-based linked list data structures I had in C++ could do that, so I rolled my own.
So as unexpected as this may be, I find a combination of map/slice, interface{}, and rolling my own meets my generics needs in Go well enough that I wouldn't want to trade off slower compilation for it, or bad compile-time error messages. I still miss generics, and it may be that the particular kinds of programming I do mean I miss them less than others. A co-worker with nearly as much Go experience as I have says he misses them more often. The ones that come up most often for me are sorting and some kind of ordered map, but even those bug me less than once a month.
Citation needed.
I've seen absolutely zero sample implementations, testing of implementation options, etc work going in with regard to adding Generics in Go.
And, aside from a couple of blog posts and shooting down people asking for it in comments, I've seen no real organized discussion, e.g. like what would go on for a Python PEP to be accepted.
Or do you think that just maybe using speed as a reason not to do something is a bit of a cop out. Especially without any benchmarks to back it up.
The Go 1.5 compiler is not slower because it is written in Go, it is slower because it was mechanically translated from another language. Human eyes will improve it over time.
Garbage collection is a throughput hit the language designers (and I) are willing to accept for improved safety and simpler APIs. I'd rather not pay it, but I choose it over spending a large chunk of my API documentation describing various ownership scenarios like I did in C++. It's a tradeoff I find acceptable for most programs. You won't find me using a GC on a clock slower than 100 MHz, or in a sub-millisecond realtime system (but I probably won't be using linux either there).
I'm also willing to pay the performance hit on non-critical generic code, and I use interface{} for that where I can. If benchmarks show it's a problem, I'll do something differently. That might be hand-rolling an algorithm, which is unfortunate for the programmer who follows me and has to read it. But it doesn't come up much.
The performance price for generating large amounts of extra code in the compiler is reasonably well understood, and not something that would be amenable to simply trying and benchmarking. One would have to implement generics, then spend several programmer-years tuning the compiler over various programs, compilers are complex machines.
Again, I haven't met a compiler expert who doesn't think that generating code for widely used generics would be expensive. And they would be widely used, any good generics solution would have to replace maps and slices, and would permeate the standard library.
