the performance for this file operation depends on the used program itself.
In particular the internal operation how files are written.
Most programs write files in blocks of bytes. Meaning, these programs collect enough data to be written to the file handle and put these out in a chunk.
This chuck will then be encrypted by Boxcryptor and written.
Some programs write their data byte by byte. As this works smoothly without any encryption applied, Boxcryptor will have a hard time encrypting and writing this file.
For each byte that get written, Boxcryptor needs to open the file handle, encrypt the byte, write the byte and close the file handle.
This is actually the exact same way Boxcryptor works with these chunks mentioned earlier, but writing byte by byte this will multiply the operation at large.
You may check this easily by writing the file to your desktop (for example) and manually move / copy the written file to an encrypted folder.
There won't be any performance issues.
Unfortunately, there is nothing we may do from our side to amend this behavior.
Thanks for your understanding.