What exactly is "several seconds"? For me it takes about 4 seconds to open the login dialog with 1000 sessions.

Thanks. I changed that setting to ranges between 100-1000 and it didn't speed up the time for the login dialog window to load. I tried a number of things including clearing the session cache but the only thing that made a difference was reducing the number of sites in bulk using a test .ini file.

As a work around I have created a script to browse the sites from the registry setting, then load the selected connection via a bat script. This custom solution loads more quickly initially within a couple of seconds but also allows me to keep it open to browse and open connections.

Perhaps there could be a way to keep the login dialog box open in the background in WinSCP? so once it's opened on the first load it can be called instantly to add/browser/change sites when there are a lot of connections?

You can try setting DontReloadMoreThanSessions to less than 1000:
https://winscp.net/eng/docs/rawconfig#dontreloadmorethansessions

When clicking New Tab it takes several seconds for the connections window to open when a lot of sites are listed (almost 1000 sites). The registry option is used for storage.

Is there anything that can be done to further optimize the connections window?