diff --git a/Orange/widgets/visualize/owscatterplotgraph.py b/Orange/widgets/visualize/owscatterplotgraph.py
index 96783a0a95d..b15d97f3abc 100644
--- a/Orange/widgets/visualize/owscatterplotgraph.py
+++ b/Orange/widgets/visualize/owscatterplotgraph.py
@@ -6,7 +6,7 @@
 
 import numpy as np
 
-from AnyQt.QtCore import Qt, QRectF, QSize
+from AnyQt.QtCore import Qt, QRectF, QSize, QTimer
 from AnyQt.QtGui import (
     QStaticText, QColor, QPen, QBrush, QPainterPath, QTransform, QPainter
 )
@@ -405,6 +405,8 @@ def __init__(self, scatter_widget, parent=None, view_box=ViewBox):
         self.plot_widget.scene().installEventFilter(self._tooltip_delegate)
         self.view_box.sigTransformChanged.connect(self.update_density)
 
+        self.__timer = QTimer()
+
     def _create_legend(self, anchor):
         legend = LegendItem()
         legend.setParentItem(self.plot_widget.getViewBox())
@@ -721,12 +723,46 @@ def update_sizes(self):
         back to widget for imputing (`master.impute_sizes`).
         """
         if self.scatterplot_item:
+            master = self.master
             size_data = self.get_sizes()
             size_imputer = getattr(
                 self.master, "impute_sizes", self.default_impute_sizes)
             size_imputer(size_data)
-            self.scatterplot_item.setSize(size_data)
-            self.scatterplot_item_sel.setSize(size_data + SELECTION_WIDTH)
+
+            if self.__timer.isActive():
+                self.__timer.stop()
+                master.setBlocking(False)
+
+            current_size_data = self.scatterplot_item.data["size"]
+            diff = size_data - current_size_data
+
+            class Timeout:
+                n_iter = 5
+
+                def __init__(self, diff_=diff, item=self.scatterplot_item,
+                             item_sel=self.scatterplot_item_sel):
+                    self._counter = 0
+                    self._step = diff_ / self.n_iter
+                    self._scatter_item, self._scatter_item_sel = item, item_sel
+
+                def __call__(self):
+                    self._counter += 1
+                    size = current_size_data + self._step
+                    if self.n_iter == self._counter:
+                        tmr.stop()
+                        master.setBlocking(False)
+                        size = size_data
+                    self._scatter_item.setSize(size)
+                    self._scatter_item_sel.setSize(size + SELECTION_WIDTH)
+
+            if np.sum(current_size_data) / self.n_valid != -1 and np.sum(diff):
+                self.__timer = tmr = QTimer(self.scatterplot_item, interval=50)
+                self.__timer.timeout.connect(Timeout())
+                self.__timer.start()
+                master.setBlocking(True)
+            else:
+                self.scatterplot_item.setSize(size_data)
+                self.scatterplot_item_sel.setSize(size_data + SELECTION_WIDTH)
 
     update_point_size = update_sizes  # backward compatibility (needed?!)
     update_size = update_sizes
diff --git a/Orange/widgets/visualize/tests/test_owscatterplotbase.py b/Orange/widgets/visualize/tests/test_owscatterplotbase.py
index e966a4fc664..34b8f9f2bf6 100644
--- a/Orange/widgets/visualize/tests/test_owscatterplotbase.py
+++ b/Orange/widgets/visualize/tests/test_owscatterplotbase.py
@@ -5,6 +5,7 @@
 
 from AnyQt.QtCore import QRectF, Qt
 from AnyQt.QtGui import QColor
+from AnyQt.QtTest import QSignalSpy
 
 from pyqtgraph import mkPen
 
@@ -16,6 +17,9 @@
 from Orange.widgets.widget import OWWidget
 
 
+DEFAULT_TIMEOUT = 5000
+
+
 class MockWidget(OWWidget):
     name = "Mock"
 
@@ -160,6 +164,9 @@ def test_sampling(self):
         master.get_label_data = lambda: \
             np.array([str(x) for x in d], dtype=object)
         graph.reset_graph()
+        if self.master.isBlocking():
+            spy = QSignalSpy(self.master.blockingStateChanged)
+            self.assertTrue(spy.wait(DEFAULT_TIMEOUT))
 
         # Check proper sampling
         scatterplot_item = graph.scatterplot_item
@@ -187,6 +194,9 @@ def test_sampling(self):
 
         # Check that sample is extended when sample size is changed
         graph.set_sample_size(4)
+        if self.master.isBlocking():
+            spy = QSignalSpy(self.master.blockingStateChanged)
+            self.assertTrue(spy.wait(DEFAULT_TIMEOUT))
         scatterplot_item = graph.scatterplot_item
         x, y = scatterplot_item.getData()
         data = scatterplot_item.data
@@ -226,6 +236,9 @@ def test_sampling(self):
 
         # Enable sampling when data is already present and not sampled
         graph.set_sample_size(3)
+        if self.master.isBlocking():
+            spy = QSignalSpy(self.master.blockingStateChanged)
+            self.assertTrue(spy.wait(DEFAULT_TIMEOUT))
         scatterplot_item = graph.scatterplot_item
         x, y = scatterplot_item.getData()
         data = scatterplot_item.data
@@ -261,6 +274,9 @@ def test_sampling(self):
                    np.arange(100, 105, dtype=float))
         d = self.xy[0] - 100
         graph.reset_graph()
+        if self.master.isBlocking():
+            spy = QSignalSpy(self.master.blockingStateChanged)
+            self.assertTrue(spy.wait(DEFAULT_TIMEOUT))
         scatterplot_item = graph.scatterplot_item
         x, y = scatterplot_item.getData()
         self.assertEqual(len(x), 3)
@@ -365,6 +381,9 @@ def test_size_with_nans(self):
 
         d[4] = np.nan
         graph.update_sizes()
+        if self.master.isBlocking():
+            spy = QSignalSpy(self.master.blockingStateChanged)
+            self.assertTrue(spy.wait(DEFAULT_TIMEOUT))
         sizes2 = scatterplot_item.data["size"]
 
         self.assertEqual(sizes[1] - sizes[0], sizes2[1] - sizes2[0])