GNU ld is indeed extremely slow, it is a very old codebase that isn't designed for modern systems, and it doesn't do any real work in parallel. GNU ld can be 2-10x slower than LLVM lld on real projects, and then mold is again ~5-10x faster that lld. The creator of mold is also the original creator of LLVM lld.
Traditionally the link time was a single-threaded step at the end of compilation, and it scaled very poorly. While compilation is embarrassingly parallel and has continued to scale. So link time was becoming a significant bottleneck, especially with incremental compilation. When you want to iterate on a large project, with GNU ld you might have to wait several minutes of link time on every dev build.