Hello, @babispanagiotakopoulos,
Sorry for the inconvenience, The thing is I have tested the slider Revolution plugin on our test site and found no issue it is working perfectly fine. To solve your issue please navigate to the Dashboard >> Porto >> Speed Optimization Wizard >> From there please uncheck the options you see in the screenshots below and then from the Slider Revolution plugin settings try to clear the cache as I showed you in the last image from below screenshots. I hope by following this practice your issue will be resolved.
If not, then please take a complete backup of your site first and then share your site wp-admin URL and access with us in the private content area of this topic so that I can check and let you know. I hope you understand.
https://postimg.cc/4n6ZdgjM
https://postimg.cc/FfXXmCDm
https://postimg.cc/VJV5Rwfg
Best Regards,
PThemes Team.