iPhone bought in Netherlands, can I use it in the USA?

iPhone Xs from the apple store online. So that means it will be simlock free right?

The iPhones sold in the Dutch Apple Store Online, are currently not part of a package deal with a mobile provider, and will be SIM lock free.

And if I buy it in the Netherlands will the internet be a lot slower in US or just a bit so that I can still use it properly?

It is not like a fixed percentage. A lot of LTE bands are compatible, but some are not. It comes down to negotiation between the phone and the cell tower. You could end up connecting to a cell tower further away, or no compatible cell tower in range at all.

E.g. Dutch networks currently use from bands 3, 7, and 20.

AT&T currently uses band 17 mainly, but also bands 2, 4, 5, 12.

Verizon currently uses band 13 mainly, but also bands 2, 4

Sprint currently uses band 25 mainly, but also bands 26, 41.

A Dutch ‘global’ iPhone Xs could work with all of that.

T-Mobile currently uses band 4 and 12 mainly, but also uses 2, 66, 71 in some areas.

A Dutch ‘global’ iPhone Xs could work with that, except for band 71. If you find yourself in a such an ‘71’ area, then your phone isn’t useable there for cellular.

Which model iPhone are you thinking of getting?

When you buy a sim-lock-free phone (i.e. not tied to a mobile provider), then you can have it work with other carriers, even in other countries.

One caveat: antenna bands that are supported vary by carrier and phone. I.e. iPhones for Verizon and Sprint are physically different from iPhones for AT&T and T-Mobile (e.g. for iPhone 8) on the 4G LTE band antenna. This has been unified a bit for the iPhone Xs and XR, but there are still North America|China|Japan|Rest of the world models that support different bands (parts of the radio spectrum). Not getting access to the preferred LTE band of your nearest BTS tower will impact the “internet speed”.

Official service on iPhone is restricted to the country where it was first sold. You would have to fly back for warranty.