C compilers became slow because of the optimizations they accreted. Plus, I imagine it just wasn't a priority for the vendors in the way it was for Borland Pascal. I'm curious how the Borland C compiler compared in speed to their Pascal compiler and to other C compilers.

Sure, not many programmers are today, sadly. I was referring to syntax analysis, or “parsing.” Some (admittedly, relatively simple) grammars allow syntax analysis to be done in one pass, so that an intermediate representation (e.g. in the form of an AST) becomes unnecessary; the latter is only needed if the grammar is complex and thus requires two or more passes for further analysis and/or machine code generation. Writing an interpreter for a command language is straightforward - exactly because analysis of its syntax can be done in one pass. Pascal is not a command language, but its grammar is (was?) still simple enough for many students of programming having gone through this (still highly recommended) exercise when Pascal was a teaching language.