I can't quite figure out what's your main question, but I'll try to answer the ones you asked as best as I can.

It seems that the type assertion is slower than the pointer cast in C?

Yes, it is. Type assertions need to be safe at runtime, thus there is a number of checks they need to perform. It's even worse with interface-to-interface assertion, because you also need to ensure that the type implements the interface.

With that said, they can definitely perform better. In fact, here is comparison of your benchmark results on Go 1.4.2. vs latest dev version of Go 1.5:

testE2I: 10.014922955s, testE2T: 4.465621814stestE2I: 7.201485053s, testE2T: 287.08346ms

It's more than ten times faster now, and Go 1.6's new SSA backend might bring even better optimisations.

And it's strange that when I use gccgo to run the same program, it would cause out-of-memory error. Does the gccgo has some limitation in gc?

-O3
testE2I: 30.405681s, testE2T: 1.734307s

Faster on the concrete type, but much slower with interface-to-interface assertion.