Help needed to fix slow site

Hi everyone!
Following is the link to a site that developed a few years back.
[link removed by moderator]
The site becomes really slow sometimes and often has a breakdown.
I have tried the options that I found online to fix it.
I think it’s the page loaders that are causing the issue. I tried to reset the index value and it worked for a little while. After that, I was back on square one.
If any of you could provide me with your precious suggestions then I be really grateful.