Read the article, 1.2 was 40% as fast on one specific benchmark, and its lack of speed largely came down to having a much worse CRC implementation, not general shortcomings.

Now 1.3 is 63% the speed of gcc O2 on this largely hostile benchmark.

More like no inlining, so-so register allocation, and a simple code generator. I believe also no loop unrolling. Other compilers could handle the table-less CRC implementation.