DoNotDetectFeatures was intended as a workaround for broken FTP servers where feature detection triggers various bugs. Disabling it might cause undesirable behavior with common FTP servers and we advice against - the increased likelihood of unforeseen compatibility issues is not usually worth the small speedup.
SupportedExtensions is not currently supported, and if added, it would open a whole new category of compatibility issues caused by mismatches between the custom-set value and the actual feature set of an FTP server. Again, this does not seem to be worth the small speedup.
However, UTF-8 can be enabled with feature detection disabled. Call
ftp.SendCommand("OPTS UTF8 ON") followed by
ftp.ReadResponse(), and if it indicates successful, set
true will speed up the first data connection a bit, and disabling classic Diffie-Hellman ciphers might speed up the control connection TLS negotiation slightly. If this is not sufficient, we would need to know what you precisely mean by a "lot of time", and a communication log at debug level showing the slowness.