Is it true that the API to create system restore points (SRSetRestorePoint) works faster on a newly installed Windows OS? It sounds logical because for couple of years the system registry (and whatever other areas are "backed up" by System Restore) grows very much. But my assumption doesn't have a background...
Can anybody provide any evidence about it or disprove it?
For Windows XP, this is true. XP approaches System Restore in a kind of simpleminded way, it copies files based on their extension. And it covers a lot of extensions.
Starting with Windows Vista, System Restore got smarter, it now uses Shadow Copy. This is a feature backed in NTFS (won't work on FAT), where files are marked as "also keep this version if someone updates the file". Time to make a shadow copy of the needed files for system restore is now only dependent on the number of files, not the size of files.
But, since the number of files that are covered (and the size of the registry, as you noted) will keep growing as the user installs more stuff, the time needed for a restore point will increase. So, the best answer would be "yes it's slower, but not much slower, unless you're on XP".
EDIT: This article describes this.