An administrative agent running at a virtualization host of a network-accessible virtualized computing service determines that a first virtual machine is to be instantiated. The agent initiates at least a first configuration operation to enable connectivity for at least a portion of network traffic associated with the first virtual machine. The first configuration operation is performed at least in part using a first virtualization offloading card of the virtualization host. The agent causes a virtualization intermediary process of the virtualization host to launch one or more execution threads of the virtualization intermediary process to implement the first virtual machine. The intermediary process may be swapped to persistent storage, e.g., based on an analysis of resources of the virtualization host.