How much multitasking you can do will be system dependent.
Burning requires a some CPU cycles, but not a huge amount. It requires a chunk of RAM, but again not that much.
Burning does require quite a lot of HDD I/O. If you have an SSD then it won't even break into a sweat, when burning and doing other tasks.
The purists will tell you, don't burn and do other tasks. Then again, the purists will tell you to buy an Â£80 HDMI cable when a Â£10 cable will do just as well.
Mileage will vary, when burning and doing other tasks. If you can multitask when burning then the quality of the burn will be same as doing no other tasks while burning.
If you get a coaster when multitasking and burning, then you would have to suspect that either your system overall isn't powerful enough, or simply that your HDD can't keep up.