Concerning the performance impact of using GuiXT with SAP GUI, can you produce some actual figures, i.e. processing time changes?

The time needed for GuiXT is normally well below 30 milliseconds.
A good way for anyone (with SAP GUI 6.20 upwards) to measure these times is the following: In SAP GUI window, at the bottom right hand side, you can display various figures, notably:

- Response time
- Interpretation time

The response time is the response time of the application server (application including database calls, total roundtrip).
The interpretation time is the front end processing time including GuiXT.

Now, you can do the following: Go to a certain screen and write down the time figures for each "Enter". Make a series of measurements to get more stable results.
- Response time: 0.850 0.650 0.550 0.600 0.590
- Interpretation time: 0.050 0.040 0.040 0.030 0.030
Now deactivate GuiXT (in GuiXT window or in SAP GUI, since this doesn't make a significant difference). Measure again:
- Response time: 0.640 0.620 0.550 0.610 0.580
- Interpretation time: 0.050 0.040 0.050 0.040 0.040

You will always notice that:
- the response time is essentially the same
- the interpretation time might increase a little bit (e.g. plus 0.02 sec).
In some cases, when you delete numerous fields in a complex screen, the time will even decrease (i.e., GuiXT saves some CPU cycles, since SAP GUI needs less time to display the screen).
The GuiXT overhead is often less than the time resolution used in SAP GUI.

Only if there are ABAP calls in the script will the interpretation time increase significantly, depending on the processing made in the called function. Or, if the scripts are on a slow network server, or web repository, the initial script loading takes some time. However, you can avoid this completely by using a local script cache.