We will soon publish an exact comparison between Shopware 6 Standard and Shopware 6 with the B2Bsellers Suite installed, but we can already say today that we have not made any relevant changes that drastically worsen the loading time. We use custom fields and repositories for extensions and have only overwritten complete functions in a few places.
An example where we have overridden a major feature is the "customer price" functionality. However, this can also be switched off if it is not needed.
Since the B2B platform is completely API-driven, the performance is of course extremely fast because there is no page change.