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Does the MacBook Pro slow down as you have less SSD capacity?
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There are varying opinions about this. Generally when your drive begins to fill up, things will slow down mainly because of the lack of free contiguous space. The system will swap virtual memory in and out to give room and that slows things down. In your situation where you still have 460 GB remaining from 1 TB, you should be okay for now.
I have many computer guy friends, some are truly obsessive. They're all PC guys, but the one thing they all insisted on was buying the most storage possible... to avoid this very issue. Add to this a very weak anecdote, but i've noticed the Macs of my own i've had the most problems with were the shortest on SSD capacity. The ones that aged better had more. They all suggested (for Macs anyways) 1Tb minimum.
This is pretty much on the money.
However in practice, you're unlikely to see performance issues manifest themselves until more like 80-90% capacity - and it depends on how heavy your WRITE workload is. Reads? They'll be fine. Writes are what will likely start to be impacted.
This is based on guidance from various enterprise array manufacturer recommendations, ZFS recommendations, etc.
This isn't an SSD or SAN problem. Its inherent to block storage. As you get close to being full, the chance of having contiguous free blocks available to write to is less. Particularly with SSDs, the SSD will also be forced to do more severe wear levelling - if you constantly run it at 90% full (e.g., you have a lot of stuff stored permnently and a small percentage of work area), it is going to need to keep shuffling blocks around in the background to spread the wear around (NAND cells have a limited number of writes they can handle) which means it is moving stuff in the background (to spread the wear so that your last 10% of cells don't get burned out) outside of the data you are trying to write to it.
Hard drives won't do wear levelling but they're so slow they will exhibit performance issues due to head movement. SSDs don't have that problem, instead they have to do wear levelling, as above.
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The 10% free someone mentioned generally makes sense as well as more or less also lining up with my experiences over the years. Systems use the disk (whether spinning platter or SSD) for virtual memory/swap space, as well as applications creating temporary files. I'm sure there are some notable exceptions here, as the former is a function of how many apps and types are all using RAM running concurrently, and the latter is most affected by sizes of temporary working files - those doing a lot of video editing may benefit from a secondary drive or at the very least increasing free space beyond these numbers.
"There are varying opinions about this. Generally when your drive begins to fill up, things will slow down mainly because of the lack of free contiguous space."
Doesn't the term "contiguous space" (which applies to platter-based drives) mean NOTHING when using an SSD?
"The system will swap virtual memory in and out to give room and that slows things down."
This DOES mean something, but there's "a way around" it:
TURN OFF VM disk swapping (can be done using the terminal).
I did this on my Macs years ago.
No crashes.
Run fine.
Doesn't the term "contiguous space" (which applies to platter-based drives) mean NOTHING when using an SSD?
"The system will swap virtual memory in and out to give room and that slows things down."
This DOES mean something, but there's "a way around" it:
TURN OFF VM disk swapping (can be done using the terminal).
I did this on my Macs years ago.
No crashes.
Run fine.
Contiguous space is meaningless but wear-leveling isn’t. Completely different issues, but for the end user they’ll perceive the same effect: a slow down.