With the right settings of the Magento 2 GTM extension, there is minimal impact on website speed. If GTM is set up correctly, the page loading speed will not be affected while loading tags.
Magento 2 GTM Extension ensures that there is no degradation in your website's user experience while you keep track of analytics and marketing tags on your website. With the right configuration, it is possible to improve site performance.
Check out more details at: https://webkul.com/blog/magento2-google-tag-manager/