Version: 5.2.1 (build 3231)
OS: Windows 7 x64
Interface: Both
Protocol: Any
Symptoms:
After connecting, WinSCP uses a constant 25% (roughly 3 billion cycles a second) of my CPU and the number of system calls hovers around 450,000 a second.
CPU usage falls to 1-2% when:

Minimized
Transferring files(using queue or not){1}
When Any dialog is open

{1} When transferring with the queue and you switch to another session tab that is not transferring, the usage goes back up.
The thread state enters and stays in Wait:WrUserRequest in all the cases.

Does not occur with 5.1.5 (build 3261)

Tools:
Process Explorer - https://technet.microsoft.com/en-us/sysinternals/bb896653.aspx
Glint - https://sites.google.com/site/glintutility/

Thanks for your report.

This bug has been added to the tracker:
https://winscp.net/tracker/show_bug.cgi?id=1013

Fixed in 5.2.2 beta and released today.