Having come from an 8-bit micro with ROM BASIC to the Amiga 1000 in 1985, I too was excited when I read about AmigaBASIC. It really seemed next gen. Unfortunately, once I started actually trying to do interesting things with it, I found it was surprisingly slow compared my 8-bit micro's ROM BASIC.
Of course even though the 8-bit was only running at ~1Mhz compared to the Amiga's ~7Mhz, the 8-bit's ROM BASIC was written in hand-coded assembler (and by some really good programmers, including BillG himself (in the case of the Radio Shack Color Computer I used)). I think AmigaBASIC must have been written in C and designed with portability in mind.
Of course even though the 8-bit was only running at ~1Mhz compared to the Amiga's ~7Mhz, the 8-bit's ROM BASIC was written in hand-coded assembler (and by some really good programmers, including BillG himself (in the case of the Radio Shack Color Computer I used)). I think AmigaBASIC must have been written in C and designed with portability in mind.