Please help us help you and fill where relevant:
Your LimeSurvey version: 6.5.2
Own server or LimeSurvey hosting: Altervista 
Survey theme/template: Fruity
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Hi! I'm writing here because I have problems with Limesurvey CE installed on my Altervista free website. Altervista started loading some pages of limesurvey very slowly until eventually the browser gave an error page, and the only way to fix this is to reload the page. Also it doesn't send the invitation emails. It was doing good, but at some point without me changing anything besides some JavaScript functions it started doing this. Also I tried to install in a new Altervista account the latest version and it gives me the same problems even without any additional code but just the default one question form.

From what I understand Altervista is a free hosting platform.

I am not surprised that Limesurvey would be slow on these websites. Servers of these services are usually completely overbooked. And Limesurvey is not a particularly "light" web application. I don't think you will get a good experience with any free or budget hoster.

Regarding emails: What are your settings for sending emails? Via PHP? This will be hit an miss, because most email servers do not trust emails sent directly via the php function. You should use a good SMTP server for that.