im thinking of buying this pass but i was told the rail passes only work on slow trains and not the high speed trains. is this true?
Not true. In Germany the pass is valid on all trains of the Bahn. In some countries, you have to pay a supplement to ride the fastest trains (Italian EuroStar, Thalys, etc), but the trains for which the pass is valid includes Intercity trains which are not slow.
In Italy, there are often a lot more EuroStar trains than ICs, practically forcing you to ride the ES. Often the supplement for the ES is significant. Railpasses rarely pay in Italy.