According to the documentation in HCL Notes 12, admins can use the notes.ini setting "NetworkSpeedInMbps" to "boost network performance" of Notes clients, when the network speed drops below a specified level.

Put "NetworkSpeedInMbps=value" into notes.ini, where value is the network speed in megabytes¹ per second (Mbps) below which network performance will be "boosted".
For example, to boost performance when the network speed falls below 8 Mbps, specify "NetworkSpeedInMbps=8". Per default, this setting is not added, so disabled by default.

"Boosting Network Performance" was a little bit to vague for me, so I opened a case and asked for more details. After some back and forth, this is what emerged:

If the Notes client notices that the network speed (calculated via the avarage time taken to send and receive date from server) falls below given speed in the notes.ini setting, "partial replication" will be enabled automatically for the mail database.
In addition, when this ini setting is set, the additionally option “Full documents if network bandwidth available” will appear in “Replication Options” of the Notes Client.

User can force the download of complete documents by selecting Actions -> Receive entire document.

Debugging settings:
TRACE_BCUPDATES=5 => All logs of this feature are added console log
DEBUG_REPL=5 => Logs replication information
DEBUG_REPL_TIME=1 => Logs replication start, end and elapsed time

which will create entries like these:

[01B8:0006-53F8:wrepl] 05/24/2021 12:20:29.07 PM CBCDCtrl::Run> Partial replication is enabled. NetworkSpeedThresholdInMbps: 8, CurrentSpeedInMbps: 56.
[01B8:0006-53F8:wrepl] Mailbox -> replication setting -> full documents.



¹ I think that should be MegaBit.