How to deal with very low performing students?
One elegant way to deal with this sort of problem (though it does take a lot of work) is to create a self-explanatory review packet (including a practice quiz!), give it out on day one, and promise that there will be a quiz on its contents during the second week of class.

You then don't need to spend another minute on the material in class, at least for that first week. (Especially for that first week. They should not feel hand-held at first! See below...) You might want to make help available during office hours, however.

This has a second benefit, which is that you can set a recommendation for withdrawal from the class. I would give this a soft sell: "if you can't achieve an 80% on this quiz, then the kindest thing I can do is to recommend that you drop the class. It's not to be mean, but you WILL NOT have a good time here if you haven't reasonably mastered these concepts."

By putting the onus of weeding poorly performing students back on the students themselves, you will (hopefully!) find that you get very little resistance. And folks who choose to stay on anyway will be making an active choice to work very hard going forward (or suffer the consequences).

The big brunt of the work need only be done one time; the review packet and the practice quiz can be the same every semester. It is only the quiz itself that need be changed.

While this should weed out some of the lowest performers (and give many others a much-needed review), if the students are as low-performing as you say, it might not hurt to touch on some of the harder concepts that people get stuck on when you come to them again in the class. 

Hopefully you'll have much better performance in the course going forward. Best of luck!