Manager expects employee to use her personal computer for work because the company's laptops are slow: 'Apparently, half my coworkers are using their own equipment because the company refuses to upgrade.'

It's not her responsibility to supply work equipment. 

It's no fun to have to use your own personal money to buy things you need to do your job. Who among us hasn't had to spend money on our own uniforms for a job, even though they were a requirement? Some might think buying a Burger King t-shirt and baseball cap is no different from spending a boatload of money on a closet full of business-casual clothes. At least in the latter case, you can use those clothes for other occasions and future jobs. If you have to spend $20 on every Burger King t-shirt that you need for work, you're a lot less likely to be able to incorporate those shirts into your wardrobe after you leave Burger King for good. 

If you have to invest in high-quality work equipment, you might be more likely to choose something expensive that will last a long time. If you need industrial work boots, you don't want to invest in ones that will last only a year. Unless your company is covering the cost, you want equipment that will last for years, so you don't have to keep going back to the store to buy new work boots. Also, investing in high-quality equipment is imperative if you're working in welding or other manufacturing or construction roles. It's surprising that some companies don't invest in good equipment, because their #1 priority should be keeping their employees safe.