No, inactive or deactivated plugins do not slow down WordPress. To understand that, let’s see how WordPress works behind the scenes.
Each time a user requests a page from your WordPress site, WordPress starts a loading process. During this process, it only loads active plugins installed on your website. All inactive plugins are totally ignored and are not loaded or even looked at.
Even if you have dozens of inactive plugins installed on your website, it would not affect the performance of your site or make it slow.
The only place where WordPress looks for the inactive plugins on your site is the ‘Plugins’ page itself. Even then, it only looks for plugin’s header file and does not load the plugins themselves.
If your WordPress site is slow, then inactive plugins are definitely not the reason.