I had just put a new SSD in my 2015 MacBook Pro and installed macOS Monterey on it. In theory, that combination should have felt faster than ever — a solid-state drive replacing the original, on a machine I knew well. In practice, the Mac felt worse than before the change: the cursor froze when opening apps, Chrome took seconds to respond to a click, and Activity Monitor showed 234 processes running at once.
The first instinct of anyone who has used a Mac for years is right on the surface: "something is reindexing". And it was right — halfway.
The first suspect: Spotlight reindexing (partly true)
After an operating system upgrade or a disk change, Spotlight rebuilds its search index from scratch. That process, mds_stores, can use a noticeable amount of CPU and disk while it runs. It's a reasonable explanation, and in the first hours after installing Monterey it was probably a real part of the problem.
The mistake I almost made was stopping there. Turning Spotlight off, waiting, or simply assuming "it'll pass" is the natural reaction when you find an explanation that sounds technical and reasonable. The problem is that sounding reasonable isn't the same as being the whole cause — and several days after the expected reindexing, the machine was just as slow.
The number that changed the diagnosis
What broke the Spotlight hypothesis was a number: the system's load average was around 80. For anyone unfamiliar with the metric, the load average doesn't measure how much CPU is in use — it measures how many processes are waiting their turn for the CPU or the disk at a given moment. A sustained 70 or 80 on an 8GB Mac isn't "it's busy", it's a sign of real saturation.
What finally confirmed the problem wasn't a specific process was sorting the process list by individual CPU usage. If Spotlight or any other app were hogging the system, it would show up at the top with a high percentage. In my case, every process showed 0.0% individual CPU — none of them, added up, explained a load average of 80. That contradiction is the clue that points straight to disk I/O: the system isn't waiting for CPU, it's waiting for reads and writes that never quite complete.
The lesson: when no process explains the problem, look at the disk
The full diagnosis — how to confirm it's disk I/O with the right commands, and how to check whether the physical disk has a real problem (SMART status) before reinstalling anything — is in the next article in this series, with the exact commands I used and what each result means.
The lesson from this first part is simpler than it looks, and it applies beyond a slow Mac: when the first visible symptom doesn't fully explain the problem, continuing to optimize on that hypothesis is wasted time. You have to look for the piece of data that contradicts the easy explanation.
The same principle applies to a website or a system that "runs slow": diagnose the real cause before investing in the wrong solution.
Frequently asked questions
Why is my Mac slow after installing a new version of macOS?
The most common cause in the first hours is Spotlight reindexing the whole disk after the upgrade, which temporarily uses CPU and I/O. But if the slowness persists days later, reindexing is no longer the cause — check the disk's health and the system's load average.
Does turning off Spotlight fix a slow Mac?
Turning off Spotlight may relieve the symptom in the short term, but it won't solve an underlying problem if the real cause is something else, such as a disk with read errors or insufficient RAM. It treats the most visible symptom, not necessarily the cause.
What does a high load average mean on macOS?
The load average doesn't measure CPU usage but how many processes are waiting their turn for system resources (CPU or disk). A sustained 70 or 80 on a mid-range Mac signals that something is saturated — almost always disk I/O, not a lack of processor power.
How do I know whether my Mac's problem is the disk and not a specific process?
If, when you sort processes by CPU usage, no single one explains the slowness (all show 0.0% or very low values) but the system feels heavy, the cause is almost always disk I/O: constant reading and writing that no single process hogs, but that saturates the disk overall.