MacBook Pro A1278 slow HDD issue

If your old drive is still operational, I recommend you try this BEFORE you remove the old drive, so that you have seen what will happen, including how long each step can take.

you can use Option-Command-R during startup to be offered either the latest macOS that is compatible with your Mac, or in some cases the macOS that came with your Mac or the closest version still available.

How to reinstall macOS - Apple Support

This article is a little bit too modern for your older Mac, but will give you the general idea:

Use macOS Recovery on an Intel-based Mac - Apple Support

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Models from about 2011 have Internet Recovery in their ROM. They are capable of booting up Utilities such as Disk Utility, even if NO drive is installed. They can also re-install MacOS without the old drive present.

If your Mac were even older, there are other techniques that can be used to get MacOS back up and running. For example, your Time machine backup drive has a recovery partition, that can be booted up to get Utilities and re-install MacOS as well as restore from backup.

In the US, many Readers have used Other World Computing house-brand drives, because they are pretty good and guaranteed to work. You specify what Mac, and they show you the drives that will work in your Mac.

https://eshop.macsales.com

Many other vendors will also work. If you tell use your market-area, we can recommend other Mac-friendly vendors.

If you do not use a Mac-friendly vendor, be sure to buy where a liberal return policy is offered.

Hi,

Also upgrade RAM makes better performance.

MacBook Pro: How to remove or install memory - Apple Support

Apple say 8GB is maximum, though you can upgrade till 16GB(two 8GB RAM stick).

To be running 10.15, yours must be the 13-in 2012 non-retina model.

Have you replaced the original drive (and cable) with an SSD drive? Those who do report, "it's like getting a whole new Computer!"

OWC can also sell you the drive cable, and replacing it at the same time is recommended. Think of it as preventative maintenance.