Am I implementing this more efficiently or not?

My employer currently uses an Access database to keep track of basically everything. Whomever originally wrote it didn't do a very good job and it's become extremely slow. I've been enlisted to rewrite it as a web-based version and I'm using PHP and MySQL. I'm currently just planning what I'm going to be doing. The main function that has caused the old database to become slow is this:

Basically what our customers do is purchase days and then they use these days. The access database keeps track of every purchase made by a customer and adds up the total days purchased and it also keeps track of every day used and they are subtracted from the total days purchased to get the total number of remaining days.

There are now well over 50,000 records and it gets extremely slow. What I want to do is to just have a single field to keep track of the number of remaining days and then when days are purchased add them to the total and when days are used, subtract them from the total. The database would also keep track of what was purchased and what was used but that information would not be used to determine how many days are unused.

I think this is more efficient but I don't know if there is something I'm not thinking of that would make this not work or become just as slow. Is there any input on maybe a better idea or something I could change or if this is the way to do it? I'd really like to know. Thanks.

Seems like an ok approach to me. It's good that you're keeping the other info, but using a simple count field might speed things up. Of course, moving off of Access and making sure you're indexing appropriately will go far toward speeding things up as well.