I believe that you verified that the sdcard read/writes are slow on the L5 due to the fact that it is on the USB2.0 bus is the limiting factor. Right? I had thought you said that wasn’t the case.
The remaining mystery seems to be why the R/W of the sdcard seems to be slower than what people here might expect on the USB2.0 bus. Some facts:
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The USB2.0 maximum transfer rate is 60MBps (480Mbps) at half duplex and, so, depending on the bulk data packet size and other protocol overhead, one might maximally get 30MBps-35MBps total.
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I believe that this limit is at the USB controller level and is shared between the devices (wifi, cellular modem). I’m not sure about the L5, but it probably has one USB2.0 controller. However, I don’t think this will be significant.
So it’s still a bit of a mystery why it is significantly slower than 30MBps. However, I think the following knowledge-base article about the particular USB2.0 hub (USB2642), it becomes more clear. I should note that I’ve seen similar performance UHS I and UHS II cards over USB2.0 so I don’t think it’s the hub in particular. Microchip Lightning Support
What SD Cards are supported with USB2642?
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USB2642 does not support UHS-I and UHS-II bus interfaces as the spec version is not compliant and the bus speeds are above the bus speed of USB2642 (35MB/s)
UHS utilizes a new data bus that will not work in non-UHS host devices. If you use a UHS memory card in a non-UHS host, it will default to the standard data bus and use the “Speed Class” rating instead of the “UHS Speed Class” rating.
i.e. This article suggests that since the USB2.0 hub doesn’t support UHS speeds … it drops to “Normal Speed” (12.5MB/s) or “High Speed” (25MB/s) [And it clarifies that this is MB, not MiB]. And, from the performance, it looks like it’s dropping to the former (“Normal Speed” of 12.5MB/s).