Improve performance of source deblending

CHANGES.rst

@@ -294,6 +294,13 @@ New Features
     are divided into chunks that are processed concurrently, producing
     results identical to the single-threaded computation. [#2407]
 
+  - Significantly improved the performance of source deblending in
+    ``deblend_sources`` and ``SourceFinder``, producing identical results.
+    The multithreshold watershed markers are now built by compiled code.
+    Deblending is typically ~7-30 times faster, both for fields of many
+    small blended sources and for large segments with many markers.
+    [#2408]
+
 - ``photutils.utils``
 
   - Added a new ``DeblendWarning`` class, a subclass of astropy's
@@ -1019,6 +1026,15 @@ API Changes
     are now computed for all sources at once in compiled code, so no
     progress bar is displayed and the keyword has no effect. [#2406]
 
+  - The ``deblend_sources`` and ``SourceFinder`` ``progress_bar``
+    and ``n_processes`` keywords are now deprecated and will be removed
+    in version 4.0, and both keywords no longer have any effect.
+    Deblending is now dominated by compiled code, so no progress bar is
+    displayed, and the multiprocessing implementation has been removed
+    because its process startup and data-pickling overheads made it
+    slower than the serial implementation at any number of sources.
+    [#2408]
+
 - ``photutils.utils``
 
   - The ``ShepardIDWInterpolator`` ``ncoords`` attribute has been