Speeding up the effect is a good idea. I’ll commit it if someone sends me a patch. Otherwise it will have to wait.

Yeah. I like this idea too. Here's a patch.

Index: vendor/plugins/tagging/lib/extend_av.rb
===================================================================
--- vendor/plugins/tagging/lib/extend_av.rb     (revision 659)
+++ vendor/plugins/tagging/lib/extend_av.rb     (working copy)
@@ -38,6 +38,8 @@
     #                       (default is :toggle_slide)
     # * +:style+            Initial CSS style on the created div (default is 'display: none;')
     # * +:text_field_class+ The CSS class to set on text fields (default is 'tag_editor')
+    # * +:effect_options+   Additional options for the effect to be passed to 
+    #                       script.aculo.us.
     #
     # The form updates the HTML added to the page by the tag_cloud_for method,
     # so you'll need to call that as well.
@@ -48,7 +50,9 @@
         :visual_effect => :toggle_slide,
         :style => 'display: none;',
         :text_field_class => 'tag_editor',
-
+       :effect_options => { 
+          :duration => 0.2
+        }
       }.update(options)

       suffix = record.class.to_s.underscore
@@ -72,7 +76,7 @@
           page << "$('tags_#{action}_field_for_#{suffix}_#{record.id}').value = '';"

           if configuration[:visual_effect]
-            page << visual_effect(configuration[:visual_effect], html_ids[action])
+            page << visual_effect(configuration[:visual_effect], html_ids[action], configuration[:effect_options])
           else
             page[html_ids[action]].show
           end
@@ -84,7 +88,7 @@
       actions.each do |action|
         form_complete = 
           if configuration[:visual_effect]
-            visual_effect(configuration[:visual_effect], html_ids[action])
+            visual_effect(configuration[:visual_effect], html_ids[action], configuration[:effect_options])
           else
             nil
           end
@@ -97,7 +101,7 @@

         html << button_to_function("Cancel") do |page| 
           if configuration[:visual_effect]
-            page << visual_effect(configuration[:visual_effect], html_ids[action])
+            page << visual_effect(configuration[:visual_effect], html_ids[action], configuration[:effect_options])
           else
             page[html_ids[action]].hide
           end