Mario,
Caching technologies like VPS, Freeform and others will only work if your RIP supports them. Check your RIP's documentation, and see if it supports one of the supported caching technologies. If the RIP doesn't support them, then it won't work if you use them in PlanetPress.
You can still try to go in the Compilation Options of your document, and try setting a maximum cache size. It might not be as effective as one of the above caching technologies, but it could result in some speed improvements.
You can also send your form to be printer-resident by going to the red button on the top left, then Send To->Printer, and download your form on the printer's hard disk. Then, you can use Watch to download the images to the printer(use the Send Images to Printer action). In your form, just call for the images's names in the picture objects(don't use a path) and don't include them in the images ressources. It should look for them on the printer's hard disk. It's not really caching, but it's still faster than loading the images from the computer's hard disk in Optimized PostScript.
Hope that helps!
Regards,
Rapha