Clean up wordpress theme -> speed up website

Hi everybody,

I have a quite annoying problem, hope someone can help me... I've made a website with a wp-theme (fiction). Everything is gone very well, the only problem is that the site is extremely slow. We did every modifications that had to be done to speed up the site (google speed test is very good; +/- 89%) but the time to load the site varies from 10 sec to 1m!!). It seems that the problem is the wp theme. Somebody can help me with that issue?

The site is www.europeansprintline.com

I would be happy to get your proposals.

Kind Regards,
Boris