Because it is sequential it will depend on how many free blocks are available. SSDs have a complicated dance between blocks that are completely empty, blocks that are partially used and need to be re-written when data is added or changed and blocks that the OS knows are not in use but the SSD currently doesn't know—i.e. Trim. If the SSD controller can't find enough free blocks it needs to run its garbage collector slowing things down significantly.
Since you are streaming smaller blocks, it is probably easier for the SSD controller to find free blocks so it is faster. Generally, you should use something like
AmorphousDiskMark to test your SSD speeds unless you specifically care about how fast you can read/write for video applications like Black Magic Resolve.