Deleting a Web Part incorrectly from the page, will not completely remove it, and the Web Part might still have some resources running on your page that will increase the page loading time.
We need to find out if we have any hidden Web Part on the page utilizing the Web Part Maintenance mode from SharePoint, so let's do it:
Edit your page, and count the total number of Web Parts that your page has,
For example, the following page in edit mode shows that it has only two Web Parts ( Events & Documents )
Open the page in Web Part Maintenance Mode, by adding ?Contents=1 to the URL of the page, and hit Enter
Once the Web Part Page Maintenance Mode is opened, it will show you a list of all Web Parts running on the page even those that you do not see ( Hidden Web Parts )
So taking our sample page, that only has two Web Parts in edit mode, in the Web Part Page Maintenance mode, we see that we have a total of 3 Web Parts ( 2 Documents and 1 Events ),
Which means we have 1 extra Documents Web Part that is hidden
In our example, we see two Documents Web Parts, but we do not know which one is the hidden Web Part that need to be removed, and which one is the real Web Part that is rendered on our page, so we will select them both, and click on the Delete link.
Then we navigate back to our page, by clicking on the Go Back to Web Part Page, to add the Documents Web Part back.
1) Why do I have Hidden Web Parts? Where did they come from?
There are many reasons for this, one of them If you deleted a ShortPoint element that contains a Web Part.
Or if you deleted the Web Part from the Edit Source button from SharePoint
2) What is the correct way of deleting a Web Part from the Page?
Whenever you want to delete a Web Part correctly from a page, follow these steps:
1. Activate the Web Part by clicking on its title, once you do this you will see a WEB PART ribbon tab at the SharePoint ribbon bar
2. Click on the WEB PART Ribbon Tab, and then on the Delete button