Open real-time systems provide for co-hosting hard-, soft- and non-real-time applications. Microkernel-based designs in addition allow for these applications to be mutually protected. Thus, trusted servers can coexist next to untrusted applications. These systems place a heavy burden on the performance of the message-passing mechanism, especially when based on microkernel-like inter-process communication. In this paper we introduce capacity-reserve donation (in short Credo), a mechanism for the fast interaction of interdependent components, which is applicable to common real-time resource-access models. We implemented Credo by extending L4?s message-passing mechanism to provide proper resource accounting and time-donation control, thereby preserving desired real-time properties. We were able to achieve priority inheritance and stackbased priority-ceiling resource sharing with virtually no overhead added to L4?s message-passing implementation. By providing a mechanism that does not impose performance penalties, while still guaranteeing correct real-time behaviour, Credo allows for the usage of microkernels in general-purpose but also in specialized systems.