Simple Google Sitemap is google sitemap generator for Opencart 2 and Opencart 3. This one is better and faster then default generator.
On a hosting with 128MB memory limit execution time is about 3 sec. for ~30000 products with ~10000 images, ~300 categories and ~350 manufacturers (if images already cached)

Features:
image tag for products
lastmod tag for products and categories
support the extension Advance News System does not support from v.2.0
the version 2.0 has its own interface and can be installed usual way
log file (execution time, IP, User Agent)


The string from log file (system/storage/logs/simple_google_sitemap.log):

2016-03-19 15:40:20 - Sitemap was generated for 0.0034 s. Request from 66.249.66.85 Mozilla/5.0 (compatible; Googlebot/2.1; +http://www.google.com/bot.html)

Extension works with OpenCart 2.x.

See also: Simple Google Sitemap Pro for Opencart 2.x and 3.x
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HOW TO INSTALL (v2.0 for opencart 2.3+)

At first, go to admin area of your site.

Step 1: Upload extension (zip file) in Extensions -> Extension installer or unzip files and upload via FTP to your site
Step 2: Go to Extension -> Feeds and Enable Simple Google Sitemap
Step 3: Find in .htaccess the line
RewriteRule ^sitemap.xml$ index.php?route=extension/feed/google_sitemap [L]
and replace it with
RewriteRule ^sitemap.xml$ index.php?route=extension/feed/simple_google_sitemap [L]

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HOW TO INSTALL (only for 1.x for opencart 2.0-2.2)

At first, go to admin area of your site.

Step 1: Upload extension (zip file) in Extensions -> Extension installer (FTP in System->Settings must be enabled) or unzip files and upload via FTP to your site
Step 2: Go to Extension -> Feeds and Enable Google Sitemap
Step 3: Find in .htaccess the line
RewriteRule ^sitemap.xml$ index.php?route=feed/google_sitemap [L]
and replace it to
RewriteRule ^sitemap.xml$ index.php?route=feed/simple_google_sitemap [L]


Support
Email for bug reports and propositions: support@helpforsite.com