Thanks for the information. Yes, the block size heavily influences the performance. As larger block sizes increase the sequential write/read performance, they also harm the random write/read performance.
Even if a single byte has to be read of the encrypted file, the whole block has to be read and decrypted by BoxCryptor because AES can only encrypt or decrypt the whole block. So if you have a (theoretical) block size of 1GB, you have to read and decrypt 1GB even if you only need 1 byte.
The larger the block size, the faster sequential operations, the slower random operations.
The smaller the block size, the slower sequential operations, the fast random operations.
A block size of 4096KB is a good trade-off for sequential and random operations.