Do I have a bad SSD? Bad block count at 257

Your SSD is in excellent shape and has no problems.

SMART attribute AA (170) is named by CrystalDisklnfo as "Bad Block Count", but other sources call it "Reserved Block Count" and define it as:

Reserved Block Count S.M.A.R.T. parameter indicates a number of reserved bad block handling. Recommendations

Although this parameter is not considered critical by the most hardware vendors, degradation of this parameter may indicate electromechanical problems of the disk.

Another description by Kingston says rather:

This Attribute is related to attribute 5: Retired Block Count. It provides a count of reserve (over-provisioned) blocks. (Note that all blocks, including reserve blocks, are in service at all times; reserve blocks constitute Flash memory space over and above the drive’s logical capacity).

The Attribute value is initially the total Reserve Block count. The value is decremented as the reserve block count diminishes over the drive’s life.

It's basically the number of good blocks that are available for mapping bad blocks, or in other words replace a bad block by one of these blocks. The number goes down, not up, as bad blocks are encountered. When it reaches zero, the firmware may place the disk in a read-only state.

CrystalDisklnfo gives as threshold the number of 10 as the danger point for your disk. I'm not sure that this number is the same as ADATA would specify for this disk, but your count of 72 is nowhere near it.

All the other attributes are excellent, with absolutely no errors recorded.

It's good practice to continue periodically checking your disks and their SMART data, to anticipate future errors.

1. What is this bad block count and is it early or later bad blocks?

Others suggested it indicates bad blocks detected while the SSD was configured at factory. Which is weird and not common practice in the past where manufacturers tended to hide such defects, so I doubt this. And I also think that if we assume 257 grown defects, then it's not reason to say "your SSD is just fine".

If we indeed would assume 'early' defects based on for example Kingston documentation, we'd have:

Raw Value Byte [3~0]: Early bad block count

Raw Value Byte [5~4]: Later bad block count

257 -> 0x [00 00 00] [01 01] -> early: 0, grown: 257.

If we however approach this from a different angle and assume 257 is remaining number of blocks reserved for reallocation then it seems to me the normalized value of 72 indicates that the drive has already been using blocks out of the spare pool, probably/possibly 28% of those.

For me personally this would be an unacceptably high number, and I would not call the drive 'perfectly healthy'.

No matter how you fly this and personal opinions on acceptable number of reallocated blocks aside, it appears the drive has been decommissioning blocks.

3. Are they excluded from I/O after being reported?

Yes.

4. Sometimes when copying a single large file, the speed drops to just 2 – 3 MB/s for a few seconds and then again jumps to 200 MB/s. Is this because of bad blocks?

I'd take a look at bad block count immediately after such an event. If number happens to increase after such an observed slow down, then yes it may very well be related. SMART values gain meaning if you can link them to such a specific event IMO.

The speed drop in itself does not have to mean anything bad.

What is normal and actually highly recommended is that your SSD has "spare" blocks that can be used at some point if some blocks on your SSD go bad. Your software may have an unfortunate choice of name for these spare blocks.

What is normal is that a brand new SSD is already using some of these "spare" blocks. Producing an SSD with 100% good blocks may be very expensive compared to one with 99.99% good blocks, and it's you who would pay the price difference.

What is also normal is that during operation some blocks turn bad and are automatically and invisibly for the user replaced with spare blocks.

What is bad is when your SSD has so many bad blocks that it runs out of spares to replace them.