Expand customer access for public website.

If your web site is accessible over the internet, then it is already accessible world wide.  Do you have code blocking people who are not in the US?

It is accessible over world wide but I am afraid there will be big slowness from the foreign side.

You should try it first.  Having two sites brings up many problems you don't have yet.  You can use one of the VPN services to access it from a different country to see how it behaves.  On the sites I support, I have not seen a lot of slowness from other countries thru VPN.

Would you elaborate what do you mean by "use one of the VPN services to access it from a different country"?

A VPN (Virtual Private Network) allows you to connect thru servers in other places and/or countries so it appears and acts like you are actually connecting to your site from there.  I use NordVPN but there are many others like HideMyAss.  There are also free ones but they tend to be overused and slow in the first place.

The web site is provided for all public access.  I do not think it makes sense to use VPN, right?

You're confusing the question.  Other people would not use a VPN.  But you can use a VPN to test how your site performs from other countries.  That's the reason I use one.

Your assumption that it would be different from other countries is basically correct but the real question is how much different?  Unless your site loads too many pages and images, it may not be that different from other countries.  You won't know until you test it and a VPN will allow you to do that.

Sorry for the confusion.  

So, I can subscribe and connect to NORDVPN, choose another country on my PC and browse the website to see performance?  Does this provide the similar effect as when my PC is physically at another country?