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Hi!
The security itself should not have an impact on the throughput . Likely something else changed, like connection interval, PHY used, etc
Could you try to capture a sniffer trace, one for the fast speed and one for the low speed case.
https://www.nordicsemi.com/Products/Development-tools/nRF-Sniffer-for-Bluetooth-LE
Could you save the traces, and upload them here?
