slow respond on next product click

Greetings.
I received feedback from customers saying when they click on a product and then click on the next button to see the next product, it takes a good 6 seconds to show the next one.
This is not at ramdom and pretty at same time of day.
I have added my dns to pagespeed of google without change.
I have activated the cache mode with 120 minutes. No change.
Would there be something else?
Site adress is mamansdeslaurentides.ca

Thank you

Hi,
1. Did you asked your hosting company about this issue ?
2. Did you tried with a different template or with your SEF module turned off ?
I think that should be some good things to do.