The battery's service life is moving towards the end, but it does not appear to be there yet. Currently, I think there are two main reasons to replace it: 1) it is swelling, or 2) you are unhappy with the current amount of battery time you get. You likely can get several hundred more cycles, with progressive loss in capacity, of course. If you are seeing a radical drop in battery performance, this could be software related. Before replacing, it is likely worth trying to reduce energy consumption via OS X settings, in my opinion. Switching to a SSD will improve your battery life, and this can be substantial over a HDD, as a SSD does not require a motor where as a HDD needs a motor that can spin a disk around 5,400 times a minute or more.
If you decide to do nothing regarding the hard drive (a suggestion I personally disagree with), it is worth being very vigilant since the drive could fail at any moment and any data not backed up will either be lost or have to go for "data recovery." Data recovery of a failed hard drive can often cost more money than a brand new computer, and it is not always successful in recovering lost data. So daily, bi-daily, or even hourly backups may be necessary to ensure you do not lose any work. Time Machine (or better, Carbon Copy Cloner,) is your best friend, since both can be configured to automatically update as often as you like, and both make good quality backups.
I agree with the Members who have noted that what you have described are "pre fail conditions" that describe a symptom that suggests hard drive failure may be imminent. Saving the document as a test does not tell us anything about the health statistics of the drive, and so I recommend considering a program like this:
https://binaryfruit.com/drivedx
the trial is free and, while diagnostic tests do NOT ALWAYS predict failure, this is a good place to start in getting an idea of what you are dealing with.
Between the pre-fail symptoms, the slowdown, and your general unhappiness, I personally think upgrading to a SSD is worthwhile and may save you a lot of frustration if you do this sooner than later. My favorite SSD that combines value + quality is the Transcend 370. Should you buy a SSD and later on the computer fail from another cause, you can always remove the SSD, put it into a $10 enclosure, and have an external SSD for use with your next computer (and if your next computer uses SATA drives, you could always pop it in and use it as the main local disk.)
If you are running OS X Sierra, and you plan to keep this system for a while, you may also want to revisit your RAM situation. An 8GB upgrade is very inexpensive, and can make a big difference in combo with a SSD if you only have 2GB of RAM currently. I'm guessing 8 GB upgrades are well under $50 these days.
Finally, have you already performed the Apple Hardware Test? What were the results?