Why can synchronous (sequential) external API calls on the server cause high latency?

Because each call waits for the previous one to finish, their times add up — the user ends up waiting for the sum of every external request, on top of your own processing.

When a request hits your server and you call several external services one after another, nothing overlaps. The total time the client sees is:

Latency = transmission time
        + total processing time
        + the SUM of all sub-requests

Concretely, if your server needs data from three APIs:

• Sequential: 200 ms + 300 ms + 250 ms = 750 ms — each call blocks until the one before it returns.

The user feels every one of those calls stacked end to end. With more services, or one slow one, the wait balloons — a bad experience caused not by your code being heavy but purely by waiting in series.

Tip: This only hurts when the calls are independent. If call B genuinely needs call A's result, sequencing is unavoidable — the fix (parallelism) applies to independent calls.