It is probably caused by the fact that SMBFS is still supporting the old SMB1 protocol only and QNAP has taken some measures to prevent accessing it via this protocol due to security concerns.

Ambient probably lags and/or is slowed down because some internal network communication inside the filesystem takes way too much time. And SMBFS can only deal with one request at a time, so it basically blocks if there are more requests incoming ...

Unfortunately, supporting a newer protocol version would basically mean a complete rewrite of SMBFS more or less - which should probably be done anyway.
I guess we need to do it at some point but it is not on our top priority list right now.

AFAICT, there is no substitute for it currently. Is there some way in your NAS to allow legacy SMB support again maybe ?