You can see for yourself what you need, and where the slowdowns are. Open Activity Monitor and we can look at a few things. I usually recommend checking it here and there to get a better understanding of your various tasks and programs, and which computer components are specifically being tapped by them:
1) CPU (tab at the upper right, if you're on macOS Big Sur; I think it used to be at the upper center on earlier versions of OS X). This one is fairly straightforward - how much is your CPU being utilized? Ignore the number for "Idle" as that just represents CPU not being used. Is your CPU utilization hitting 100% or near it when you're running some of your tasks, particularly when you're experiencing slowdowns? If so, then you're CPU-constrained. Most people blame their CPU for the slowdown, but I'd gauge it to be a less frequent cause of slowdowns, particularly for beachballs.
Let's move on.
2) Memory. Look at the bottom and you'll see a bunch of things. The simplest graphical representation is the memory pressure graph, which should tell you if you're being memory-constrained or not. You can look at some other numbers for more. Swap Used indicates things that could not fit in RAM (or that your computer decided to remove from the RAM, for what ever reason) and that reside on your hard drive - ideally the swap should be very low, and rarely used. App Memory and Wired Memory are the two big numbers to look to. You can also do a search on the process list for Lightroom, if you don't see it (search magnifying glass is at the upper right) and get a sense of how much RAM Lightroom, itself, is using.
If your RAM is bursting to the limit and/or your system is showing memory pressure in the red, then 8 GB is definitely not enough for you. If memory pressure is in the yellow then you'd likely benefit from more but it may not be critical. If your "swap used" is in the multiple gigabytes then you very likely have a memory problem. (For reference, I've had my computer running for one week since its last reboot, I've done some photo editing and virtualization; on a 32 GB system, my "swap used" is around 900 MB.)
3) Skip over to Disk.
I use a different utility for monitoring this one, but if you're running most of your work from your main drive (and not an external) then you may still be able to use Disk Utility for this purpose. To make this one useful, from the upper menu (upper left of your screen - not the app), go to View > Update Frequency > Very Often (1 sec). The default is a bit too slow. Run your tasks and look at both the read ins/sec and writes out/sec (left of the graph); data read/sec and data written/sec (lower right of the graph). A standard 7200 RPM HDD can be expected to do something around 400 for read in/out, or read/write speeds of 80-160 MB/s (not combined, but singularly). If you're hitting those numbers then it means that your hard drive is working at its maximum speed, which may or may not be causing you problems. Admittedly, it's rare to hit maximum speed for a sustained period of time. If you're
not hitting those numbers, then you need to see what's happening with your CPU and RAM to figure out what's going on, because it could mean one of two things:
a) something else is slowing down elsewhere, and your hard drive is idling while waiting for those things (rare);
b) your hard drive is having to do a lot of seeking to find data, which wastes time. This is extremely common, and much more likely if your swap file is large, or if you're trying to run a lot of tasks at the same time.
Even if you couldn't get comfortable with the Activity Monitor, know that for the vast majority of people, replacing their hard drive with a SSD results in a much faster, much more responsive computing experience. As you may have noted above, there is an intersection between RAM and the hard drive in the form of "swap memory," and a faster hard drive (in this case, a SSD) will make swap utilization much less noticeable.
Your system cannot use the latest and greatest SSDs of today. They're about 2-4x as fast as the SSDs you
can use, but the SSDs you can use are still 5-10x as fast as your current HDD, depending on what metric you're comparing. The prices have become much more affordable, as well. For example,
a Samsung 1 TB model for $120, or if you'd prefer to save a bit of money and don't mind sacrificing a small bit of performance and also don't care for this to last for decades,
Samsung's QVO line 1 TB for $99 (note: I have the 8 TB QVO connected over USB, as well as a Samsung 1 TB NVME drive connected over Thunderbolt, and differences in speed for my photo work are largely academic).
If you already have a SSD in your MacBook Pro then I would guess that there's a RAM limitation, and I'd agree that it may not be worthwhile to make that upgrade at this time. But if you're still running the stock hard drive that came with your computer, I'd bet you that swapping it out with one of the SSDs I linked above (or even an alternate) would give you another 1-2 years of life on your current system, if not more. It's not a bad investment, since you can take the drive back out of your computer and use it as a fast external later on. If you like the current M1 offerings then make the upgrade, but if you're still wanting to hold out for more software transitions or more hardware iterations, then the SSD would be a nice way to buy you some time without having you pull your hair out over all the beachballs.
Utilize the Activity Monitor to prove to yourself what you'd benefit the most from.
