adding more Additional nodes (we call them extensions) causes your KNIME to load slower (similar to how more addons in your browser cause it to start up slower). I would only install the extensions that you need.
You can uninstall by going to the the toolbar and click Help > About KNIME AP > Installation Details. Select all the extensions you don’t need and click the Uninstall button at the bottom of the dialog.
Some extensions are installed by default and I would advise to keep them. Here is a screenshot of all extensions in a minimal KNIME 3.6.2 installation.
You might want to check your virus scanner. On certain systems with quite aggressive virus scanners I experienced KNIME to be slow. Since it can contain several 1,000 single files an ‘agressive’ virus scanner might scan all of them every time they are accessed/opened. When we changed the settings KNIME started faster. Of course there is a trade off between security and speed. But modern virus scanners might be able to have a flexible configuration.
But of course the number of AddOns and the overall performance of the machine also play a part. Considering overall performance you might want to have a look at this thread:
If you want to have great models I would encourage you to keep the H2O.ai nodes though