I saw this same problem after upgrading to 8.x-1.29, with searches taking several minutes to complete.
After applying the patch the performance has returned to normal.
🐛 | Search API | PHP 8.1 preg_replace(): Passing null to parameter #3 ($subject) of type array|string is deprecated HtmlFilter processor
I have just updated search_api from 8.x-1.25 => 8.x-1.29 and ran into this problem when I tried to rebuild the index.
One of the problem pages has an image pasted directly into the text, so the html is:
<p><img alt="" src="image/png;base64,iVBORw0 - LONG HEX STRING FOLLOWS
It has the alt="" which matches the original description of the problem.
I've attached a copy of the contents of $value that was passed into function processFieldValue()