As a temporary fix, I've disabled the com_content tagging functionality in core files by commenting out <!-- < field etc> --> the tags < field > in:
/administrator/components/com_content/models/forms/article.xml
/components/com_content/models/forms/article.xml

The article edit pages now load a lot faster but would obviously prefer not to use a core hack. If there could be a core com_content option to disable that field, for now for those with a lot of tags, that'd be great.

Actually ajaxSearch is already implemented, but not used as you've noticed the whole list is preloaded. I had similar issue, with 100,000 tags slowing down the browser. Please test my PR.

Thanks, that looks awesome, will check it out!

For temporary fix you can also:

diff --git a/libraries/cms/form/field/tag.php b/libraries/cms/form/field/tag.php
index 7c8cc04..19cd1fe 100644
--- a/libraries/cms/form/field/tag.php
+++ b/libraries/cms/form/field/tag.php
@@ -140,7 +140,7 @@ class JFormFieldTag extends JFormFieldList
                $query->order('a.lft ASC');

                // Get the options.
-               $db->setQuery($query);
+               $db->setQuery($query, 0, 30);

                try
                {

and then you get only first 30 tags. After you type 3 chars you get next max 30 tags.

yes, I used that temp fix myself, before finally diagnosing the issue to characters and implementing the proper fix.

Hi CSThomas, your temp fix doesn't really work reliably for me, I think - it loses pre-existing tags when you edit a new article.

Sandrodz's fix sort of works (though it sometimes seems to struggle with finding longer tags or tags with spaces in them when you have a lot of tags on the site). Overall, great job Sandrodz - sorry you're stuck with the Unicode bug - I would help if I could, but Unicode is damn complex...

@desigit

Try below:

diff --git a/libraries/cms/form/field/tag.php b/libraries/cms/form/field/tag.php
index 7c8cc04..365e231 100644
--- a/libraries/cms/form/field/tag.php
+++ b/libraries/cms/form/field/tag.php
@@ -139,8 +139,17 @@ class JFormFieldTag extends JFormFieldList

                $query->order('a.lft ASC');

-               // Get the options.
-               $db->setQuery($query);
+               if (!empty($this->value) && is_array($this->value)) {
+                       $query->where('a.id IN (' . implode(',', $this->value) . ')');
+
+                       // Only load selected values.
+                       $db->setQuery($query);                                                                                                               
+               }                                                                                                                                            
+               else
+               {
+                       // Get first 30 options.
+                       $db->setQuery($query, 30);
+               }

                try
                {

Second block 'else' will be use for articles without tags or for tags from popup.

If someone interests I have changed above patch a little.

This is a temporary path for joomla that does not show any tag options in searchtools and in modal batch.

diff --git a/libraries/cms/form/field/tag.php b/libraries/cms/form/field/tag.php
index 7c8cc04..708f57a 100644
--- a/libraries/cms/form/field/tag.php
+++ b/libraries/cms/form/field/tag.php
@@ -139,12 +139,21 @@ class JFormFieldTag extends JFormFieldList

                $query->order('a.lft ASC');

-               // Get the options.
-               $db->setQuery($query);

                try
                {
-                       $options = $db->loadObjectList();
+                       if (!empty($this->value) && is_array($this->value)) {
+                               $query->where('a.id IN (' . implode(',', $this->value) . ')');
+
+                               // Only load selected values.
+                               $db->setQuery($query);
+                               $options = $db->loadObjectList();
+                       }
+                       else
+                       {
+                               // Do not load any options - temporary fix.
+                               $options = array();
+                       }
                }
                catch (RuntimeException $e)
                {
diff --git a/libraries/cms/html/tag.php b/libraries/cms/html/tag.php
index d44636e..16075f6 100644
--- a/libraries/cms/html/tag.php
+++ b/libraries/cms/html/tag.php
@@ -136,7 +136,7 @@ abstract class JHtmlTag
                $query->order('a.lft');

                $db->setQuery($query);
-               $items = $db->loadObjectList();
+               $items = array(); //$db->loadObjectList(); // Temporary fix

                // Assemble the list options.
                static::$items[$hash] = array();

Closed as this is resolved in #9155 which needs tests