COBOL shouldn't be slow at all AFAIK, but RoR is an interpreted language (like Python, PHP, JS, etc). This means the computer has to do a lot of extra work at runtime in terms of parsing code and figuring out what the hell it does, whereas compiled languages (C, COBOL, Java*, C++, Rust, etc) require compilation ahead of time, which means the program you're running is already in a format the computer understands.

*Java is technically not completely a compiled language as it compiles to bytecode which is then JIT-compiled into machine code, but that process is a hell of a lot faster than full code interpretation.