Double Speed rewards advance the Progress Bar at twice the speed as usual. Every time you play at Chipz Casino, your Progress Bar advances a bit. Playing with Double Speed makes it advance twice as fast.
This leads to faster Level-Ups, which means more personalised Free Spins rewards.
Keep an eye out for future Triple Speed rewards.
General Double Speed rewards are usually only valid for 1-2 hours, whereas game-specific rewards may be active for several days.