SFTP - files are only transferred in short bursts for one client - routing issue or SFTP/SSH configuration?

I have a server running Ubuntu 21.04.1 LTS with daily incoming file transfers via SFTP. For the most part, these work fine, but there is one particular client where the transfer speeds are unreasonably slow.

Using a python script to monitor the growth of the incoming files, I noticed that the transfers come in bursts, rather than continuously. For example:

2023-01-10 10:57:50 --> 900.94 MiB (0.00 B/s)
2023-01-10 10:57:51 --> 900.94 MiB (0.00 B/s)
2023-01-10 10:57:51 --> 900.94 MiB (0.00 B/s)
2023-01-10 10:57:52 --> 900.94 MiB (0.00 B/s)
2023-01-10 10:57:52 --> 900.94 MiB (0.00 B/s)
2023-01-10 10:57:53 --> 900.94 MiB (0.00 B/s)
2023-01-10 10:57:53 --> 901.47 MiB (1.06 MiB/s)
2023-01-10 10:57:54 --> 902.11 MiB (1.28 MiB/s)
2023-01-10 10:57:54 --> 902.11 MiB (0.00 B/s)
2023-01-10 10:57:55 --> 902.11 MiB (0.00 B/s)
2023-01-10 10:57:55 --> 902.11 MiB (0.00 B/s)
2023-01-10 10:57:56 --> 902.11 MiB (0.00 B/s)
2023-01-10 10:57:56 --> 902.11 MiB (0.00 B/s)
2023-01-10 10:57:57 --> 902.11 MiB (0.00 B/s)
2023-01-10 10:57:57 --> 902.11 MiB (0.00 B/s)
2023-01-10 10:57:58 --> 902.11 MiB (0.00 B/s)
2023-01-10 10:57:58 --> 902.11 MiB (0.00 B/s)
2023-01-10 10:57:59 --> 902.11 MiB (0.00 B/s)
2023-01-10 10:57:59 --> 902.11 MiB (0.00 B/s)
2023-01-10 10:58:00 --> 902.11 MiB (0.00 B/s)
2023-01-10 10:58:00 --> 902.11 MiB (0.00 B/s)
2023-01-10 10:58:01 --> 903.24 MiB (2.26 MiB/s)
2023-01-10 10:58:01 --> 903.26 MiB (38.04 KiB/s)
2023-01-10 10:58:02 --> 903.26 MiB (0.00 B/s)

As you can see, there are almost 10 seconds of nothing between two bursts of data transfer. For other clients, the transfers are pretty much continuous:

2023-01-10 11:21:14 --> 30.00 MiB (703.31 KiB/s)
2023-01-10 11:21:14 --> 30.41 MiB (831.19 KiB/s)
2023-01-10 11:21:15 --> 30.81 MiB (831.19 KiB/s)
2023-01-10 11:21:15 --> 31.34 MiB (1.06 MiB/s)
2023-01-10 11:21:16 --> 31.75 MiB (831.19 KiB/s)
2023-01-10 11:21:16 --> 32.28 MiB (1.06 MiB/s)
2023-01-10 11:21:17 --> 32.72 MiB (895.12 KiB/s)
2023-01-10 11:21:17 --> 33.16 MiB (895.12 KiB/s)
2023-01-10 11:21:18 --> 33.72 MiB (1.12 MiB/s)
2023-01-10 11:21:18 --> 34.15 MiB (895.12 KiB/s)
2023-01-10 11:21:19 --> 34.72 MiB (1.12 MiB/s)

My questions:

1. Is this a normal pattern?
2. Could this pattern give us any indication as to where the bottleneck lies?
3. Are there any settings that can be changed on our server to counter this bottleneck?
4. Provided I had access to the client machine (which I don't have at the moment), how could I further troubleshoot the connection other than running traceroute?

No, that pattern isn't normal. (Although using a python script to monitor the growth of the incoming files is not a good gauge. You should monitor the network bandwidth towards your server, filtered by source address.)

We can only speculate but the most probable cause is some bandwidth policing at their firewall. A true bottleneck would more likely show itself by a low overall transmission rate, not bursts like that.

If you have access to the client machine you can do some download/upload testing, to see whether yours is the only site impacted.