SSDs have a limited number of writes before they stop functioning. My guess is that you’re getting close to that after 6 years. SSDs are less expensive now than in the past.
I don’t know if you can defragment an SSD though I don’t know why you wouldn’t be able to do so. Still, my guess is that the SSD is reaching the end of its useful life and should be replaced. A new SSD will likely be significantly faster than your current one when it was brand new 6 years ago as well so you’re going to get a significant speed boost.
From my understanding, fragmentation is not an issue on an SSD. Indeed, data may be intentionally fragmented when written, as part of wear-leveling. In my “museum” of old apps is iDefrag. Its developer said that defragging an SSD was pointless and even counterproductive. Between SSDs and the emergence of APFS, he closed up shop. I still have some external HDs formatted as HFS+, so I still break it out once in a while–but, I think, for more sentimental than practical reasons.
The scan report shows you have a MacBookPro11,1 model, which still has a user-replaceable SSD. I have not done a 2014, but my 2012 was very easy to replace the SSD. I did it twice. (Once to 512GB, then to 1TB)
My favorite source for internal drive replacements is Macsales.com and internal drives for your model are here. No vested insterest; just customer of theirs numerous times over the last decade.
An internal 480GB bare drive is currently $139 with “speeds up to 3282MB/s read and 2488MB/s write” which is significantly faster than you have right now. You can also get up to 2TB internal for your model. You can also get a kit version with tools and an enclosure to hold your new drive during a copy, and your old drive when done for about $80 more. Or just do a time machine backup to an external drive, install the bare drive with cheap compatible screwdrivers (if you don’t already have them), replace the bare drive and restore from time machine.
Since that machine is Thunderbolt2, you’ll get better performance by replacing the internal SSD then using an external drive. I believe (but could be wrong) that your external speeds may be limited to 960Mbps or so. Which is still significantly faster than you have now, but the internal change is easy and not that expensive.
This is another very good point. Buying a larger than anticipated need SSD not only allows for better future growth, but lets the SSD optimize the wear patterns of sectors better. In fact, if the current SSD is “nearly full” (intentionally vague) that can contribute to slower speeds. I am not sure how you define “nearly full”.
For a 2014 era MBP, I’d also probably replace the battery while you have it open. Or at least use a utility like coconutBattery to check the battery health and charge capacity. (The app is not free on MAS, but is free on many download website if you do a web search.)
I stated that I removed more than 50GB of data from that disk (to an external HDD) and reboot many times because I read somewhere that the OS (macOS) do the job for me (a.k.a. start writing from the very first free SSD block).
As usual, macOS works hard at shut down time (check the shut down time after a simple / short session, then to a complex (with many small downloaded files) and 24 or 48 hours session.
At last, I check with the Apple Utilities (Disk Utils) application once a week.
Thanks all for your answers.
I will work for some days with an external HDD / High Sierra until next week and try MacWay (Strasbourg Shop), ask them for costs (replace hardware and job doing that).
If that was a 2.5" HDD, I would have done it by myself (and I already do that), but I do not even have the correct tool to remove the back plate :(.