It depends heavily what the API calls are doing in the backend. Node.js despite handling requests asynchronously is single threaded. If your logic includes synchronous operations, the overall performance is bound by the execution time of these operations. Start by measuring how the system performs with 1 API call/s, 5 API calls/s and so on. If 1-5 calls/s are very fast and with more calls becoming slower and slower, you definitely have a bottleneck in the backend. Next thing to do, is to build in some logging to measure where your system is spending its time.