Tardis is correct. Your brake is not working properly and needs to be fixed.
Every car I have ever owned has been able to completely lock the rear tires by applying the parking/emergency brake. This is how it ought to be. If you pull the handle all the way up, the tires should be totally unable to turn. Period. They should skid on the pavement before turning. When you performed your test and pulled the parking brake at 25MPH on a flat road, there should have been people running out of their houses to see the squealing tires and burned rubber.
Granted, you need to really man-handle that thing to get the wheels to lock. But if you reach the end of the handle's range of motion and those rear wheels are still turning, that is a problem and should be fixed.
There's clearly a widespread myth out there that this brake is somehow only supposed to be used when the car is already parked. Yes, you'd normally never ever use this brake to actually stop the car, and yes, you primarily use it to hold the car in position while it is parked. But think about it, if it doesn't bring the car to a stop while it is moving, then how can it possibly have enough holding power to keep the car from rolling down a hill? Simple answer: It doesn't!
Lots of peoples' parking/emergency brake is in poor repair because of several factors: 1) Steel cables are used to operate the parking brake, instead of hydraulic lines. Steel will stretch and rust over time, giving you less effective braking power. 2) Rear drums have an auto-adjuster which is supposed to keep the pads near the drum surface. But this auto-adjuster can freeze and no longer extend the pad to make up for wear. 3) Automatic transmissions with a prawl on the output shaft have made many think that the parking brake is redundant. (which is true to some extent, but does not eliminate the need)
So, yeah - take your car into a shop and have them tighten up the cable. As long as other stuff is in good repair this should be a simple fix.