Hi Dave,
What is the your user-exit on logon doing? Maybe it is calling a front-end service which is somehow failing? Could you please check if SAP Note 1787341 - "ABAP Interface Restgui" is applied, and if not, please apply it and test again without your parameter.
Now let's talk about performance. If there is a problem with latency then there is not really anything we can do. I assume you feel the same impact when using the SAP GUI for HTML. My only recommendation is to look into scripting when personalizing your screens because this allows to reduce the amount of roundtrips which usually has quite an impact if you have a high latency.
Cheers,
Tobias.