Will it be slow if I use only one HTML page?

I designed and coded my own website, however, it is just one simple html page and it contains links to different sections ( I used display: "none" and when user clicked on a menu option, I hide the previous and show the other ). It contains about 6000 lines of code since its about 5 pages stuffed in one. If I host this single HTML page, will it be alot laggy? especially during alot of traffic?

My CSS stylesheet and Javascript file is separate though

Speed would not be my primary concern, though you would be making everyone download everything at the beginning, which is something to avoid when you can.

What would concern me more is that nobody can bookmark the section of page that they are interested in. Also search engines may not be able to index your content as well with so many topics covered on the same page.

For most web sites, it's best to split information into multiple pages if you have any doubt that the page is too long.

It depends on the overall size of the file. Anything around 100-200 kb is fine.