Performance of script access: When a user calls a transaction, e.g. VA01, the first time, the GuiXT script is downloaded from SAP Web Repository to the user's PC. Next time the same user calls VA01 the script is read from his PC, which is now differently configured. Can there be a difference in performance reading the script from the local PC, caused by how the PC is now configured?

There is no performance risk reading GuiXT scripts from local files, even when you use numerous 'include' files. If you have a problem with slow response times, you have to look for other issues, e.g. processing too many screens in an InputScript.