A client wants to use NetSuite as a sort of backend for their about to be built website. They want all their product categories, subcategories, price levels, item info, dimensions, customer info and even product images come in from NetSuite. Is this even doable with RESTlets? I can see performance being poor in this scenario.

Yes, it can be done with either an externally facing suitelet, or a restlet. If you use a restlet you will need a server between Netsuite and your frontend which has authentication credentials. The suitelet is easier to setup IMO but it would depend on your use case. I haven’t had any issues performance wise.

I see, thank you, It's good to know performance wise it can be done. I was leaning towards RESTlets because I'm more familiar with them but it seems an external facing Suitelet is the best option here. Why is an auth server needed when using a Restlet but not with a Suitelet?

Because a restlet cannot be made “externally facing” it always requires an authentication token with each request.

That can be a hassle sometimes.

If you set it up incorrectly, yes. On the suitelet you should run it under a role which has view-only permissions on just the record types needed for the website. Then specifically define the endpoints on the suitelet to expose only the data needed for the website.

@Mr_Tib what is the website built on? You can integrate any out of box or custom records via REST API, we have done this with Magento 2.