If I remember correctly the configuration values to tune are:
Download.MaxWriteBufferPerDownload = 262144
Download.MaxTotalDownloadWriteBuffer = 1048576

Values are in bytes. Defaults are 256K and 1MB

You should be able to set them in phexCorePrefs.properties.
Check phex.prefs.core.DownloadPrefs for documentation of the values.

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