The major version bump is for some incompatible changes, though they are unlikely to cause trouble.

• keyword arguments to psort besides :key have been replaced with a single :granularity argument; the old arguments are now ignored
• removed deprecated aliases from 1.2.0 and 1.3.0 (you may not be aware of them since they haven’t been listed in the documentation)
• A function defined with defpun is now optimized for N worker threads where N is the number of cores. The old behavior is available with defpun*, which defines a function that is optimized for N-1 workers (and has less overhead).
• added psort* — like psort but targets N-1 workers
• improved performance of psort
• task categories are now compared with eql; same for ptree node ids

The benchmarks page has been updated to reflect recent optimizations, which includes optimizations in SBCL itself.