A multi-FPGA System called Flow-in-Cloud (FiC) is currently being developed as an accelerator of Multi-access Edge Computing (MEC), which is one of the core 5G technologies. FiC is a system that consists of multiple mid-range FPGAs tightly connected by high-speed serial links. Since timing-critical jobs are assumed to be executed in MEC, a circuit-switched network with static time-division multiplexing (STDM) switches has been implemented on FiC. In this paper, we investigate the methods to enhance the interconnection performance of FiC. We proposed the link aggregation and the slot distribution for making the use of multiple lanes naturally supported with Aurora-IP and economical FireFly cables. We evaluated combinations of the techniques by using three practical applications on a real FiC prototype with 24 boards. When the number of slots is large and the number of packets is small, the slot distribution is sometimes more effective, while the link aggregation is superior for other most cases. As a result, the communication performance could be improved up to 3.01 times by using four lanes. Furthermore, we found that application designers can use enough resources even using four lanes and the link aggregation is better than the slot distribution.