MacBook Pro has a slow start-up.

My MacBook Pro (Model Number: A1278, Serial Number: C1MM44B2DTY3) has a painfully slow start-up. Whenever I log in, it takes forever for the computer to start running smoothly. I've tried to avoid shutting the computer down and instead put it to sleep to avoid the issue, but as I'm doing something on the computer, it starts to crash and freeze. I've tried removing any start-up apps, but still the problem persists. I also have about 400 GB of free storage, so it can't be because of low storage. I have no idea what to do. Please help!