I don't think it has a service ceiling, specially since it is magic that keeps it afloat.
As for speed, there is only one way to find out. You'd have to fly so fast that ablation due to air friction becomes an issue. The rider would be in danger way before you attained that speed anyway, so unless you are planning to do a sketch on Jackass, you shouldn't worry about that.
Here's what you should worry about:
Being stiff (probably due to all the cheicals you put on it to protect your property against ticks, fleas and silverfish, and to keep it impermeable), I surmise it will bank to turn. The traditional picture that comes to mind when we think about magical carpets allows for VTOL, but for some reason I am thinking it has to pitch to go up. Given these characteristics, even for slow, tranquil flight I would suggest you sew some security harness onto the thing. Lawnchair Larry knew that safety comes first when it comes to flying on your furniture.
Also beware that the friction with air will charge the carpet and its passengers with static. This may have all sorts of effects and hazards, so be careful with electronics and discharge safely whenever you land.
Fire is also an important safety hazard, so the use of narguile's on the carpet should be strictly forbidden according to international aviation laws.
Some people may have carpet allergies (seriously), so be sure to always have some antihistamines on you for your passengers.
Last but not least, if you have to take a pet along with you, be sure it is in a box. Not only because their claws might damage the carpet.