Has anyone worked with any 3rd party web stores that integrate well with NetSuite? We're B2B and orders tend to have 100+ lines; our customers have complained about SuiteCommerce's performance to the point where they'd rather enter an order over phone or fax.

Typically for B2B with 100+ lines I have seen some form of EDI ordering portal. I have personally used SPS commerce. It was a pain to setup but worked well for our B2B customers. There might be other options too.

I've used Shopify syncing via Celigo for B2B, and found it cost effective, easy to set up & reliable. Shopify isn't innately suited to B2B in terms of e.g. design & variable pricing, but most things are possible with a bit of development. I've used this setup with B2B customers ordering 1-200 lines alongside group+item pricing, with no real performance issues and customer feedback generally positive. Would recommend.

shopify, magento

Have you tried Orderful for EDI? Happy to chat through