I use codebook on Windows 10 Version 10.0.18363 Build 18363 on a Lenovo ThinkPad T470s running an Intel i5-7300U CPU @ 2.60GHz with 12GB of RAM.
For some time I have the problem that Codebook and the Secret Agent lag and slow down or freeze. Most times they usually still respond to the keyboard and mouse, and sometimes I can still copy+paste passwords and things, but it is incredibly sluggish. Other times they are so sluggish or completely unresponsive that they are unusable. The level of degradation and usability in this state seems to vary by some unknown variable.
This behavior only seems to occur when a Windows RDP (Remote Desktop Connection) session is active. Usually, the RDP has been in use (i.e. opening a fresh RDP does not degrade Codebook or Secret Agent). I remote into another machine, then call Secret Agent to inject a password, which is when I notice the degradation.
Closing the RDP window resolves the lag/slowness/freezing and Codebook/Secret Agent returns to normal. Often, I close all my RDP windows, re-open them, and I’m able to use Codebook smoothly for a short time before it starts to degrade.