5ba5968 created the issue

i7-3820QM, 32Gb RAM
Windows 10 Pro 10.0.18362 Build 18362
Cyberduck v7.0.2
MountainDuck v3.1.0

Physical interface is dedicated to the Hyper-V switch, host OS is using virtual one via the "Internet sharing" feature:

> Get-NetAdapter | select Name, MediaType, MtuSize

Name                       MediaType     MtuSize
----                       ---------     -------
Physical                   802.3            1500
vEthernet (ext_switch)     802.3            1500
vEthernet (Default Switch) 802.3            1500

I have noticed that both MountainDuck and CyberDuck use excessive CPU while using protocols that impose small penalty on the traffic.

For example, unencrypted FTP transfer over the 1Gbps localnet:

The same using MountainDuck:

and its threads, sorted by CPU usage:

Lastly, I have noticed that MountainDuck seems to cache content at system drive.
Can this be disabled so application writes directly to a destination?

While I am not quite familiar with NT-based systems, I am more than happy to help with the debugging of this issue.