Consider deleting any unused taxonomy tags
As a part of keeping your Drupal site clean and manageable, you should look at removing any vocabularies that have zero terms. Also, you should look at deleting the terms that have no content. Having terms with no related content can end up causing your site to have a lot of pages that are public and have no content. To search engines like Google this looks like your website has multiple pages with duplicate content and can have a impact on how your site will rank. Additionally, having a lot of terms can clutter up the admin dashboard and make managing and selecting terms difficult.
Consider deleting any unused content types or consolidating similar content types
Similar to Vocabularies and empty Terms, you should audit your Drupal site's content types and look to see if there is a way you can consolidate them into fewer types. This allows your site's content creation team to be able to create content easier and have a small set and defined list of content types that they can choose from when trying to communicate your message to users. Another upside of having fewer content types is again it will allow you to have a smaller set of items that you need to maintain and clean up the admin interface for those items that have an important role for you marketing needs online.
Disable any unused views
Along the lines of cleaning up and simplifying you should review all of your views on your Drupal site and disable or delete those that are no longer used. Like empty terms, this can create pages on your site that are public but have no content or very stale content. One of the big things about ranking well for Google is making sure that content is updated regularly and that you are not showing blank pages to Google. Like other steps, this is a good housekeeping step that should be done regularly or after updates to your Drupal website.
Simplify your navigation
You should review the navigation for your Drupal site frequently to make sure that it is as simple as possible. The ideal goal is to have it set up so that users can find any piece of content within two or three clicks. The fewer steps your user has to take to get to the content that they are most interested the better. This keeps them more engaged, reduces the likely hood of having a high bounce rate, and increases the chances of them converting to a paying customer instead of just a visitor.
Consider removing the slideshow
Slideshows used to be a really big deal for websites just a few years ago, but with a bigger focus on providing websites that are accessible to everyone and the fact that often times they can be frustrating to users the time has come to start considering using Hero Images instead. Selecting one or two well-suited hero images that relate to the content on the current page has been shown to have a better impact on converting users to customers than a traditional Slideshow does. There may be some easy ways that you can reconfigure your slideshow into being a platform for a hero image instead. If your slideshow was built in views then you may be lucky and can just change the number of items to show to just a single item. Most Slideshow libraries are smart enough to hide navigation elements when there is only one result.
Check for broken links
Like anything, it is important to make sure that your content has references to good resources. You should schedule a regular time to go through all articles that have off-site links and check to see if they are still good and if they are still the best possible reference available for your content. Many times just changing what you are linking to can have a really good impact on your Google search placement for your articles. Additionally, linking to dead links not only can hurt Google, but it also can hurt your website's image for users. If users find frequently that your links are not good then they will stop using your site as a reliable resource which could have a harmful impact on your conversions.
Check website accessibility
Accessibility is a complex topic and can be hard to get right the first time. Many countries are now looking at passing laws that require websites to be accessible to the Visually impared. This means you have to think about how you are rendering images, controls, and links on your website so that they can be read and controlled by a screen reader or other similar device. Aside from that you also need to look at the colors you use and the font sizes that you use on your website so that if visitors need to change the font size or if they have some form of color blindness they can still easily view your content