SSD suddenly very slow

Actually, I think you need to write all ones to the drive.

But seriously. What is being suggested is a massive forced TRIM operation. This suggests that TRIM is not properly enabled on your system, and the drive is not getting TRIM commands. You need to ensure that it is getting TRIM commands. What OS are you running, what motherboard or make and model of PC, what mode is the SATA controller set to in BIOS?

If the SSD is not getting and processing TRIM commands, it will think that all of its pages are full and spend an enormous amount of time copying dead data over and over to make free pages ("garbage collecting"). All internal to the SSD's controller.

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The process above should be done as follows.
1) Download and burn to CD something like the Parted Magic distro.
2) Do a complete image backup of your SSD. Test it. Be paranoid and restore it to a spare HDD and try to boot off the HDD. Once you believe that you have a good backup,
3) Boot the Parted Magic or other distro and do a "secure erase." A utility to write all ones, or all zeros, will be a disaster on an SSD. The Secure Erase does a factory reset, and guarantees that all of the space is marked as free.
4) Restore the backup to the SSD.

I always do a Secure Erase before doing a restore to an SSD. Otherwise, it gets stunk up thinking that old data still needs to be protected.

Remember, do a Secure Erase, not an overwrite utility.

If it's a Secure Erase, then it should work just fine.

I have a question about troubleshooting this problem in general; I'm not disputing your recommendation. How do you know that the drive has to be zeroed out versus needed the Samsung performance restoration utility? I did read the referenced thread " Samsung 840, sudden very poor performance. 32MB/s write and 100MB/s read. " The article I pasted below is worth reading. By the way, I used to own several Samsung 840 EVO SSD's. Before they released the performance restoration utility, I defragged my SSD and that work-around worked for months to hold me over.

I stopped trusting the 840 EVO as I believe Samsung made too many engineering missteps. I backed-up my drive with Acronis (Clonezilla would probably work fine too) and I deleted/recreated the a partition on the drive and did a full format (not quick). Then I sold the used drive on Ebay after having installed the latest firmware on the drive as a courtesy. Then I purchased the 850 EVO.

Samsung 840 EVO Performance Restoring Firmware Only Partially Effective
http://www.pcper.com/reviews/Storage/Samsung-840-EVO-Performance-Restoring-Firmware-Only-Partially-Effective

I don't know that, it's just my top guess based on cases I've seen and read about. Could be the fault of the 840.

You might be completely right. Either way, this issue is fascinating. Thanks for the info.

In my case the performance restoration utility worked. My problem was that the read speeds were very slow, but I don't believe my write speeds were affected. It's very interesting reading your statement of the problem in this thread, because I also thought my slow computer was do to an issue with my internet connection. I knew of the problem with some Samsung 840 EVO's that had data that had been stale for 6-8 weeks, but it didn't occur to do that I was a victim of this problem. Strangely I no longer see this utility being offered. I could be wrong, buy my guess is that Samsung has integrated this tool into the latest Samsung firmware. Your Samsung Magician states that you have the latest firmware?

To answer your question: Defragging my 840 EVO's data resolved the problem because then the data was no longer stale. This story references the problem that I experienced: http://www.pcper.com/reviews/Storage/Samsung-840-EVO-Performance-Restoring-Firmware-Only-Partially-Effective . I'm not sure if secure erase will resolve your problem or not. Your issue sounds different than mine because mine was with read speeds not write. But a defrag would be a easy way to find out. I run windows 7 professional. I haven't touched Windows 8.1 or 10. For all I know, that drive has technical hiccups with Windows 10. I'm honestly not sure, but the timing is suspicious.

Sorry that my response is so verbose and unfocused; I just woke up. Since you're only dealing with a 250 GB drive, personally, I'd just buy a new Samsung 850 EVO 250 for US$100 off NewEgg and call it a day. I would install Windows 10 or whatever OS I wanted on it, copy my data over and once I felt secure that it was up and running, I would do a secure write on the 840 and then sell it on Ebay. That's what I would do, just to eliminate the drive model as a cause. I just don't like long term uncertainty in my computer builds. I've have a single 850 EVO 500 GB in my Lenovo X230 laptop; that's the computer that I used to have a 840 EVO 500 GB. In my workstation, I used to have 4 x 850 EVO 250 GB in RAID 0. I never had any problems with my 850's with the exception that the Samsung Magician software doesn't recognize drives in RAID, but that's a known issue. I simply updated their firmware prior to placing them into RAID and it was fine.

Defraggler is a good program.

http://filehippo.com/download_defraggler/

* click the green "download latest version" button on the top right of the page.

OMG NO! Never defrag an SSD! Stop!

http://www.pcworld.com/article/2047513/fragging-wonderful-the-truth-about-defragging-your-ssd.html

It puts wear on the SSD and does absolutely nothing useful. Defragging an HDD reduces seek times. SSDs don't seek.

Check if firmware is EXT0DB0Q if not do a upgrade. Magician allows to make a performance optimization, which is kind of manual TRIM. Would try this first. Current Magician ist 4.6 www.samsung.com/samsungssd