Hi, Wim.
You're allowed to increase sim rate when you're flying off-line(XAcars only). When you're using a network(VATSIM, IVAO, FSEconomy) the rules of the network precede.
A search on our own forum told me you can use time compression on VATSIM but you have to tell ATC at the start of your flight.
Please, please look at your XAcars report before sending it. I've read mixed stories about XAcars handling time compression.
Of course using 2 airports in between 2 points and is also an option. Then you can simply skip the boring part in between.