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Released June 2012 / Core i7 processor with Turbo Boost / Up to 1 GB GDDR5 Video RAM

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Slow MacBook Pro (15in, Mid 2012, Catalina, SSD, 16GB)

Hi all, I have loved using this laptop for the last 10 years but in the last 12months it seems to have become slower and slower :(. I have found Firefox becoming very slow when browsing Facebook and other sites and Thunderbird and Keynote can be incredibly slow - complete stop. Occasionally copying files from one destination to another on the SSD or from a USB drive or SD card can also become extremely slow. I generally fear multitasking on this machine now

It has two SSD drives (Operating system and Data) I recently wiped both SSDs and re-installed Catalina from scratch before re-installing Thunderbird etc. I think it helped a little with the general speed but there are still times as mentioned when things grind to a halt.

My one thought is to free some space up on the 2TB SSD. Thank you for any other help and suggestions.

EtreCheckPro Report:

Update (10/09/23)

Question about replacing the HDD cable:
Every guide I find tells me that the correct model number for a replacement HDD cable for my mac is:
But when I opened the Mac to check what is inside it has: 821-1198!

Block Image

From what I can tell this is the incorrect cable for this Mac but as far as I am aware the cable has never been replaced! I am the second owner but the first owner was elderly and barely used the machine and I think I bought it from him after he had owned it only around one year.

My Mac has the following info:

Order MD103LL/A
Model A1286 (EMC 2556*)
Family Mid-2012 15"
ID MacBookPro9,1

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Your cable is the older version which has issues running at 6.0Gbps SATA III you’ll want to replace it.


@danj That is great to know. Do you have any links to more info on this? Did Apple include these cables on the original mid 2012s?


@adambendror - I’m not following you here? When a part is rev’ed or made by a different vendor one often marks a part ID of some sort to it can be identified: old Vs new. Your cable is the original cable and the 1492 is the newer.


@danj I am just surprised that the 1198 is in my machine when it seems like 1492 is the correct cable for the machine. I'm trying to figure out whether Apple would have included the 1198 with this machine or whether someone incorrectly installed it later?


@adambendror - I can’t see that as being logical. Why would someone put in the older cable if it had the newer? This system is just likely one of the first shipped which had a SATA II (3.0 Gbps) drive Vs the newer SATA III (6.0Gbps).

So… running this system with a SATA III SSD you need the better cable.


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There are two possibilities here! The first since you have two SSD’s installed is not using the HD SATA port to hold your boot drive. This is important in the older Unibody’s as the Optical drive SATA port has some issues as the boot drive. So I would start off checking. The second is the HD SATA cable tends to break down when moved about to much. I would inspect it and you might just want to replace it. Here’s a bit more on the problem Your Hard Drive Cable Is A Ticking Time Bomb while the 15” model was less prone, over stressing the cable can damage it. The thin foil wires within the ribbon can get damaged by over bending fatiguing the wire causing issues when moving data.

The best way to think of it is trying to run a race car down a dirt road instead of a nice smooth road. While the race car might make it through it will take longer!

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Great tips thank you. I have had my OS drive in the optical drive for years without an issue and have had my data drive in the HDD slot. Upon further investigation I am finding write speed to the data drive (2TB) can become extremely slow at times (under 10MB/s). This all leads me to three things:

1) time to replace the SATA cable

2) time to check that the 2TB drive is still functioning optimally

3) maybe I dont need two drives anymore. It used to make sense to have a small SSD and large HDD but now SSDs are more affordable maybe I can get away with having a single drive in the machine again.


@adambendror - if the optical drive has a fixed SATAII (3.0 Gbps) speed drive you’ll get much better thought on it! Most SSDs today are fixed speed SATA III (6.0 Gbps) which won’t play nice in the optical drive.

We had tons of issues with dual drive setups when we jumped to SATA III drives. Between the drive cable issue just with spinning iron drives HDD, and more so with SSDs.

We finally went to single 1TB SSDs and did a few 2TB when they first came out. We where supporting about 750 MacBook Pro’s.

Just swap the drives so the boot drive is in the HD bay, you will need to use the startup key Option (⌥) to then select the boot drive which is now in the other bay. Then alter the startup drive setting in your prefs.

Mac startup key combinations

I think that should fix things!


@danj Thanks again Dan!

I will take your advice and move my OS to the drive in the HDD bay.

I have a question about replacing the cable though. I cant post images in this reply so I have posted the images and question below. Would you be able to take a look?


I almost had it solved!

- Installed brand new 2TB SSD and installed in the HDD slot.

- Reinstalled Catalina

- Replaced Cable with 821-1492

Black Magic was giving read and write speed of 500MB/s consistently.

I started recovering my data and around 300GB in noticed a lag. Went to Black Magic and found the same error. Sometimes getting great read / write speed and then at times randomly dropping to 10MB/s or below.

My one thought is that the 821-1492 cable that arrived looked second hand (missing the cover over the adhesive) and I had to bend it to fit it. I am thinking to get another cable - make sure it is brand new and install it in such a way that it doesn't have to bend as severely.

Besides this any other suggestion? Could it be that my machine doesn't like a Crucial 2TB SSD? Also do you suggest that I have TRIM enabled?

I have to say that overall the machine is snappier - more responsive. But every now and then I can see the disk speed drop way down and it starts slowing down


@adambendror - Unlike HDDs, SSD’s as they get denser can get too hot which does effect the I/O speed. So heavy write or read events can appear slower at the high point as the SSD controller will slow things down, which in turn slows what ever is going on within your system.

So can one improve the cooling? Yes! And that’s what I would do using a good thermal monitoring app like TG-Pro it allows you better info on what’s happening so you can see what actions are needed if it’s something deeper. It also allows you to ramp up the fans so more heat can be expelled from the system.

Sadly, the 2.5” SSD case cover does reduce the cooling of the flash chips. Without getting too deep here the denser the flash chips the heat threshold drops! So a 2TB drive will get hotter sooner than a 1TB which uses the same generation of flash chips just less dense version.

The generation of flash chips can also effect things as the older had more surface area for the heat to dissipate better (4 chips Vs now often with 2) the I/O speed with fewer chips also reduces as like RAID multiple drives speed up the I/O as one can span across.

If you are someone who is normally doing heavy I/O types of work (picture/video rendering) then I would remove the drives cover and install a fin type of heat sink to offer better heat transfer. But even that only goes so far in a laptop!


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So,it's because ur computer is getting old.Itvwill only make it a little fast if you free up space,I just reccomend getting a new one.If only apple used smart switch .-.

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