Be very careful in this situation. Not only is it fully legal for them to fire you for not meeting performance standards, but they can also probably keep you from being approved to draw unemployment benefits if they've carefully explained the work load to you and gone over the work expectations, and you've met the requirements before. In other words, progressive discipline where you know that your job is in jeopardy if you do not meet the goals and have elected not to try your best.
In this situation, it is more likely that the "being set up" you sense is trying to get you to quit. If you do that, you have a much smaller chance of being approved for unemployment insurance, which costs the company money and they do not like your getting to draw it. If you quit, you are less likely to draw. Much less, in fact. If they have to fire you, it's a bit easier for you to be approved.
But your best plan of action is to keep on doing the best you can, looking for other jobs while you are still working there. Tell them, pleasantly, if rebuked about your productivity, "I am doing the best I can do." Make sure it is true. Do not goof off or let them see you showing attitude or slacking in any way, whether you are being treated fairly or not.
If they call you in and terminate you, do not sign anything saying you have resigned or agree to quit. Do not jump up and say You can't fire me, I quit. Just leave quietly, and go home and file at once for unemployment insurance. In filing your claim and talking to workers from the unemployment agency, make heavy use of this phrase, "I always did the work to the very best of my abilities." You'll begin to make weekly certifications, and in several weeks, they'll give you a decision on whether or not your claim is approved.
Yes, it is legal for the employer to set the work performance standards too high to be achieved and use this as an excuse to terminate any employee they wish to terminate. You live in an "at will" state, where you can be terminated "at will" for just about any reason, very very few of those reasons being illegal or something you could sue them for. Unemployment insurance is possible if you have the monetary eligibility and if it is determined you showed up and did your best and still could not perform the work adequately. If fired without a valid misconduct reason you stand a good chance of being able to draw unemployment for a while as you are looking for another job.
One thing you may want to look at and be ready to answer questions about is "Is your workload higher than that of your co workers? Were they able to meet these productivity goals?" and "Was there ever a time when you met those goals? Have your work productivity goals been raised significantly?" and be SURE to mention you were doing your work to the best of your abilities. Nobody can reasonably sell fifty vacuum cleaners a day, nobody can spin straw into gold, and they can legally ask you to unload seventy pallets all by yourself. (Or in an hour, if they want to.) It will come down to unemployment being your only recourse and them making a decision on whether or not your work load was something the company could reasonably expect you to get done.

