In most assembly languages, accessing local variables on the stack is easy, plus you have multiple registers for temporary data. Forth feels extremely limiting compared to that.
On an architecture without those features, like the 6502, Forth may be a good idea, and possibly faster than C - but only if it's compiled to machine code with some peephole optimizations, so that e.g. "123 MY-VAR C!" translates into "LDA #123 ; STA MY-VAR", instead of a naive implementation where the address and constant would first be pushed onto the stack.
And any more complicated optimizations would probably require first "decompiling" the Forth code back into a higher level of abstraction. It's practically the same as assembler macros otherwise.
edit: fixed order of operands. I originally wrote "MY-VAR 123 C!", but then remembered that the address to store to has to be on top of stack. IMHO, infix notation is less confusing, and writing a recursive-descent parser to handle it isn't that hard compared to everything else in implementing a compiler. And of course in an infix language, "123 := MYVAR" would be a syntax error, instead of storing (the low byte of) the address of MYVAR into memory location 123.
On an architecture without those features, like the 6502, Forth may be a good idea, and possibly faster than C - but only if it's compiled to machine code with some peephole optimizations, so that e.g. "123 MY-VAR C!" translates into "LDA #123 ; STA MY-VAR", instead of a naive implementation where the address and constant would first be pushed onto the stack.
And any more complicated optimizations would probably require first "decompiling" the Forth code back into a higher level of abstraction. It's practically the same as assembler macros otherwise.
edit: fixed order of operands. I originally wrote "MY-VAR 123 C!", but then remembered that the address to store to has to be on top of stack. IMHO, infix notation is less confusing, and writing a recursive-descent parser to handle it isn't that hard compared to everything else in implementing a compiler. And of course in an infix language, "123 := MYVAR" would be a syntax error, instead of storing (the low byte of) the address of MYVAR into memory location 123.