SSD’s are much faster than older hard drives that have spinning platters. However there are a few issues to be aware of. The explanations below are simplified to be more understandable, but the general idea stands. This normally applies to NVME SSD’s, but also the SATA SSD’s. USB drives with flash memory often have very slow performance for other reasons.

Bad Performance due to low free space: A 1TB drive which is 99% full (only 10GB free space) will typically perform much worse than a drive with much more free space. When writing data to a drive, it needs to have empty cells in order to write data. When deleting files, Windows will tell the drive that the areas it used are no longer needed, but the drive usually won’t immediately erase them, it just marks them as ready to be erased. Also, if the SSD controller is busy it might not even mark them as available, if this happens then it won’t know it can reuse that space until a Retrim command is sent by Windows (typically once per week). Writing a bunch of data on a drive that is nearly full can have a huge slowdown due to these and other factors.

Drives which are mostly full die faster. A 1TB drive which is 99% full (only 10GB free space) will typically die about 50 TIMES FASTER than a drive which is only 50% full. This is because most SSD controllers won’t move existing data around on the drive preemptively, meaning all temporary files and writes get concentrated in that 1% of free space which wears it out. Most new consumer drives are rated for each cell to have around 1,000 writes, and so concentrating them on 1% of a drive means failure could be months instead of many years.

Drives running slowly after a while. Cells in a SSD are like tiny batteries (capacitors). An example is if the voltage in that cell is 1 volt it may contain two bits of data, 11. If the voltage is 0.65 volts then it contains 10. If it contains 0.35 volts then it is 01, and if it is 0 volts then is is 00. That gets you four distinct voltage levels in one cell, which lets you store two bits of data, four cells combined is then 8 bits of data which is one byte of storage. The capacitors slowly lose voltage over time, just like a battery slowly loses power, even if you aren’t using it. After a year or so (much faster if it is hotter) then that 1 volt may have drained down to 0.7 volts. If you read that you might think it is 0.65 volts that was just a tiny bit higher instead of 1.0 volts that has drained down a bit. Drives do have some extra checksums which can correct a certain amount of bits changing, but if the controller doesn’t proactively rewrite that data (erasing the cells which lowers the voltage to 0 in every cell) and then writing the data back (putting the right amount of voltage back), then it gets harder and slower for it to read the data.

Some facts about SSD’s:
– Individual cells have limited writes, usually around 1,000 writes before it degrades enough to not be trustworthy.
– Writing data requires that cells are empty first, and so changing a file basically erases it and writes it over again.
Writing data happens in chunks (called Pages), typically 8KB at a time. This means changing a single byte actually writes about 8KB of data.
– Erasing data usually happens in Blocks of around 256KB to 2MB.