Short answer – no.
The Fixel script is loaded asynchronously (only after your site has finished loading), so it will not slow down your site.
The script’s size is a mere 15KB. We even optimized our script to score 100/100 on Google’s Speed Test tool so that its impact is as minimal as possible.
Additional Fixel events fired are based on the user’s behavior and these too happen in the background without impacting the user’s experience.