Ask the teacher for a few copies of the timed tests that they do and then go and make a bunch of copies for yourself to use at home. Start by cutting them apart and having her only do 10 or 20 in a minute (or whatever) and then add more and more. I would simply just cut the paper after the last problem that you want her to do. You can save the rest of the page for practice or recycle it, but if you start breaking it up, it will be a more attainable goal.
I would hesitate giving any type of punishment unless you feel she really CAN do it, she's just playing "games". I sense that is not the issue. If she really is trying her hardest, don't punish her, math just might not be her thing. Rewards are good and I know you said you did some and it didn't work, but maybe if you make the rewards based on the "easier" tests that you are creating at home, it might spark some more interest. Or...make the rewards based on her doing well at her flash cards. If she can do X amount in a minute, she gets a sticker towards her goal of X treat (ice cream, a small toy, etc.)
You could switch it up and do the subtraction facts, but honestly, the plus are way easier and if she has those down pat, the subtraction are that much easier.
Also, you could have her make some flash cards out of note cards or something. Maybe having her write them will help commit them to memory. Don't do them all at once, but maybe 5-10 at a time.
Finally, I would start playing as many games (board games, card games, etc.) that have to do with numbers and counting. The more numbers she sees, the more comfortable she'll get. Uno is a good one, dominoes, etc.