When building public projects on the Performance Plan, all builds will use paid credits. This document (https://circleci.com/docs/2.0/oss/#overview) states customers are given 100,000 free credits per week for their open source projects, but this is only applicable to users on the free plan. This is clarified here: https://support.circleci.com/hc/en-us/articles/360038512113--How-does-CircleCI-charge-credits-for-Private-and-Public-Repositories-
Users who are building on open source and private repositories will count against the number of user seats available for the billing cycle.