Those compilers don't do nearly as much optimization as LLVM does. It's hard to overstate how wide the gap between the classical compilers you like and LLVM/GCC truly is. LLVM has no fewer than three IRs, each with a full suite of compiler optimizations (four if you count ScheduleDAG). LICM, just to name one example, runs at each stage (see MachineLICM.cpp). Turbo Pascal did nowhere near that level of optimization, and it produced unacceptably slow programs by modern standards.