In practice, flash based SSDs basically never do a full read-modify-write cycle to do an in-place update of an entire erase block. They just write the new data elsewhere and keep track of the fragmentation (consequently, sequential reads of data that wasn't written sequentially may not be as fast as sequential reads of data that was written sequentially).
RMW cycles (though not in-place) are common for writes smaller than a NAND page (eg. 16kB) and basically unavoidable for writes smaller than the FTL's 4kB granularity
So if large sequential writes mean that you only write full whole blocks, that can be done much faster than writing the same data in random order.