My MBP is very slow

Reinstalling macOS would be harmless, but it's not justified for the circumstances you describe and probably won't help.

Please read See how apps affect Mac performance, battery runtime, temperature, and fan activity - Apple Support.

If that does not reveal anything of interest, then proceed with the following:

Reproduce the events that cause your Mac to exhibit that problem. During that time, capture two screenshots as shown below, and post them in a reply.

Please refer to the CPU section in Use Activity Monitor on your Mac - Apple Support.

In Activity Monitor's View menu, select Active Processes (not "My Processes" as shown in the screenshots).

Click the CPU tab, and post a screenshot similar to the following, with the %CPU column sorted in decreasing order.

Then, click the Memory tab and post a screenshot similar to the following, with the Memory column sorted in decreasing order.

To learn how to post a screenshot please read the Appendix in Writing an effective Apple Support Communities question.

Please don't post any information that you might consider personal. The above User Tip includes a link explaining how to edit the screenshot, if needed.

The actions you describe strongly support a conclusion that the problem is not caused by software, and the screenshots you subsequently posted indicate your Mac's performance is not being caused by a lack of RAM.

I have reason to believe that the slow startup and poor performance is likely to return. The fact that macOS runs well on an external hard disk drive supports a conclusion that the internal one is operating in a state of failure.

Replacing it is easy, especially since you already installed macOS on the external one. If and when the poor performance returns you can simply swap the two drives, and your problems are over.

I confess that I did not view your videos, although I will do that when I have more time. Your description itself is more than adequate.

Back up your Mac if you have not done so already. To learn how to do that please read https://www.apple.com/support/backup/. To learn how to use Time Machine read Use Time Machine to back up or restore your Mac. The reason for creating a backup is that the symptoms you describe could be indicative of an impending hard disk drive failure, so securing your data is a primary concern.

Then, shut down your Mac and restart it in "Safe Mode". To learn how to do that please read Use safe mode to isolate issues with your Mac - Apple Support. Compare how your Mac works in Safe Mode to how it has been, and describe your observations in a reply to this Discussion.

Then, restart normally. If the same problem returns, please read the section If an issue doesn't happen in safe mode for Apple's recommended actions.

Finally, you can temporarily create a new User Account, and log in to that account to determine if the problem also exists when logged in to that Account. To learn how to do that read How to test an issue in another user account on your Mac. When you are finished troubleshooting, you can remove the temporary User Account by following these instructions: macOS Sierra: Delete a user or group. Before removing it, confirm you don't need any of the files you might have created in that Account.

Let me know what you determine.

Thanks for the videos. They are very helpful. Your Mac is so slow at performing nearly every task that it is nearly unusable.

I doubt that reinstalling macOS will be worthwhile.

The symptoms you describe and the actions you took strongly suggest a failed internal hard disk drive. A related logic board fault is another possible cause, but far less common than inherently life-limited traditional hard disk drives.

It's very important that you create a backup before doing anything else. Your existing Time Machine backup is about nine months old. I strongly recommend that you set that one aside, because you probably will not be able to use it to create a new backup without erasing the existing backup first. (If you were to do that, it would temporarily leave you with no backup. Given the symptoms exhibited by that Mac, there is a significant likelihood that Time Machine will take a very long time to finish, if it finishes at all). An additional external hard disk drive of 1 TB capacity will be sufficient.

Since Sierra introduced a completely new logging system, I do not know of a reliable way to diagnose potential hard disk drive problems. Disk Utility will not suffice. Until I come across a disk in a state of failure yet operational enough to test I can't provide a means of extracting any applicable log entries that could definitively show those failures. A definitive conclusion could be drawn by installing Sierra on an external hard disk drive, booting from it, and determining if the symptoms are alleviated while running Sierra from that startup disk. To do that read How to set up and use an external Mac startup disk - Apple Support. If the external hard disk installation of Sierra works properly while the internal one does not, that is all the justification you need to replace the internal drive.

Hard disk drives are inexpensive and are easily replaced. The ability to restore from a Time Machine backup makes their inevitable failure a non-event.

You might want to consider adding some more RAM. Using modern OSs, 4 GB of RAM is about the bare minimum.

The 2 places I’ve seen recommended most to buy reliable RAM are below. I have purchased RAM several times from Other World Computing and have always been very satisfied with the product and service. They have on-line instructions on how to replace the RAM. OWC has also tested RAM above what Apple states is the maximum. I had 6GB installed on an early 2008 iMac supposedly limited to 4 GB and noticed an improvement.

Crucial

Other World Computing

Hi Babloo,

The reason I suspect a failed hard disk drive is based on the symptoms you described. Although operating system corruption is certainly possible, when nearly every action you take (such as logging in, moving files or folders, launching apps and other routine user interaction) results in frequent and prolonged appearance of the "wait cursor" then it suggests the Mac is having difficulty moving information to and from its hard disk drive — a task all Macs routinely perform regardless of the amount of installed memory.

When the same problems occur in "Safe Mode" it eliminates the effects of installing non-Apple software that can interfere with normal operation, and you confirmed that to be the case.

It would help if you could capture the screenshots of Activity Monitor such as I suggested, but accomplishing even that basic troubleshooting task appears to be difficult on that Mac. For example, if that Mac's "Memory Pressure" graph is frequently "red" it would indicate a deficiency of RAM given the applications you use. More memory is generally beneficial, but unless you can demonstrate a deficiency of RAM is the sole cause of poor performance then it is not worthwhile adding any more.

Given what you have done so far, it is still impossible to definitively implicate the hard disk drive as the sole cause, but it is a common cause since all rotational hard disk drives have been cheaply built for years. When they have been in service as long as that one appears to be (five or more years) their failure rate begins to gradually increase, especially on portable Macs that are subject to the routine and unavoidable environmental abuse required of portable equipment.

Installing macOS on a separate, external hard disk drive and booting from it will help confirm or refute the diagnosis of a failed internal hard disk.

Your HD is getting pretty full.

You need a more recent TM backup if that is your sole source of insurance.

How to create a boot clone