Working a long time with MS RDP classic client (so called “Remote Desktop Connection”, mstsc.exe), previously I never knew about this configuration option, found in
VirtualBox troubleshooting chapter:

RDP client collects input for a certain time before sending it to the RDP server. The interval can be decreased by setting a Windows registry key to smaller values than the default of 100. The unit for its values is milliseconds.

100ms is a pretty noticeable delay.

The key does not exist initially and must be of type DWORD. Depending whether the setting should be changed for an individual user or for the system, set either of the following.

HKEY_CURRENT_USER\Software\Microsoft\Terminal Server Client\Min Send Interval HKEY_LOCAL_MACHINE\Software\Microsoft\Terminal Server Client\Min Send Interval