A lot of 3rd party services such as Google Analytics require you to add scripts to your head tags in order to work.
This usually isn’t an issue when you only have 1 or 2, but can really slow down your website when you have a lot of them.
If you have multiple scripts you can optimize their delivery by using a tag manager, such as Google Tag Manager. This will mean you only need to add 1 script directly to your website, so they’re fewer requests to load.
Also just like plugins, not all scripts are created equally, so some might slow down your website more than others.
To see which scripts are having the biggest effect, run your website through GTmetrix, then click “waterfall” and have a look to see if any of the scripts have large load times.