A slow site loses people who were ready to read or enquire. That is the conversion problem. A perfect speed score on a page with no offer does not create a lead.
Keep templates light, images sized, and the important content visible without waiting on extras. Measure the pages that receive campaigns, not only the homepage.
Treat speed as part of the build, then watch whether enquiries move. If they do not, the offer or the form is the next place to look.