OK, so I solved this by editing CUTIL.e
In AddDotNetTagFile()
_str msg = 'The .NET libraries have not been tagged yet.
To do that now, click <a href="<<cmd gui-make-tags">here[/url].';
//_ActivateAlert(ALERT_GRP_WARNING_ALERTS, ALERT_SYMBOL_NOT_FOUND, msg, "Tagging", 1); // JPORKKA REMOVED!
I removed the call to _ActivateAlert().
This seems to be caused because I do not have .NET libraries installed, and Slick gets upset.
In a test here, I found it called this function 167 times when starting Slick!
It appears then that Slick keeps track and tries to display that toast 167 times, and takes quite a while to do so.