Append layout objects without rebuilding the existing array

add_annotation, add_shape, add_layout_image and add_selection appended
with `self.layout[prop] += (new_obj,)`. That re-validated every existing
element into a fresh copy, and each copy looked up its own props with an
O(N) identity scan, so one append cost O(N^2) and N appends O(N^3).

_append_array_prop validates only the new element and keeps the
existing ones. Batch mode keeps the old path.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: jayzhou2309

CHANGELOG.md

@@ -8,6 +8,7 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 - Fix concurrent first access to lazily initialized graph object properties, which could raise `ValueError("Invalid value")` [[#5691](https://github.com/plotly/plotly.py/pull/5691)], with thanks to @hb1915 for the contribution!
 - Fix `mpl_to_plotly` crashing on touching bars (such as `plt.hist`) due to floating-point noise producing negative `bargap` values by clamping `bargap` to `[0, 1]` [[#5696](https://github.com/plotly/plotly.py/pull/5696)], with thanks to @robertoffmoura for the contribution!
 - Fix `px.sunburst`, `px.treemap` and `px.icicle` listing sectors in a different order on every run when `path` is used with a Polars DataFrame; sectors now follow their order of first appearance for all dataframe backends [[#5766](https://github.com/plotly/plotly.py/pull/5766)], with thanks to @Irahan2 for the contribution!
+- Fix `add_annotation`, `add_shape`, `add_layout_image` and `add_selection` rebuilding every existing layout object on each call, which made adding N objects take O(N^3) time (1000 annotations: 25 s → 0.05 s) [[#5779](https://github.com/plotly/plotly.py/pull/5779)]
 
 ## [7.1.0] - 2026-09-15
 

plotly/basedatatypes.py

@@ -1627,7 +1627,7 @@ def _add_domain(ax_letter, new_axref):
             xref, yref = map(lambda t: _add_domain(*t), zip(["x", "y"], [xref, yref]))
             new_obj.update(xref=xref, yref=yref)
 
-        self.layout[prop_plural] += (new_obj,)
+        self.layout._append_array_prop(prop_plural, new_obj)
         # The 'new_obj.xref' and 'new_obj.yref' parameters need to be reset otherwise it
         # will appear as if user supplied yref params when looping through subplots and
         # will force annotation to be on the axis of the last drawn annotation
@@ -5369,6 +5369,38 @@ def _set_array_prop(self, prop, val):
         self._compound_array_props[prop] = val
         return val
 
+    def _append_array_prop(self, prop, val):
+        """
+        Append one element to a compound array property
+
+        Unlike `self[prop] += (val,)`, this validates only the new element
+        and keeps the existing elements, so N appends take O(N) time
+        instead of rebuilding every existing element on each append.
+
+        Parameters
+        ----------
+        prop : str
+            Name of a compound array property
+        val
+            The element to append
+        """
+        if self._in_batch_mode:
+            self[prop] += (val,)
+            return
+
+        curr_val = self[prop]
+        validator = self._get_validator(prop)
+        (new_el,) = validator.validate_coerce([val], skip_invalid=self._skip_invalid)
+
+        self._init_props()
+        new_dict_vals = self._props.get(prop, []) + [deepcopy(new_el._props)]
+        self._props[prop] = new_dict_vals
+        self._send_prop_set(prop, new_dict_vals)
+
+        new_el._orphan_props.clear()
+        new_el._parent = self
+        self._compound_array_props[prop] = list(curr_val) + [new_el]
+
     def _send_prop_set(self, prop_path_str, val):
         """
         Notify parent that a property has been set to a new value