ecryptfs may cost some memory, but without measuring it I'm quite sure it's negligible in comparison to your 4GB. It shouldn't cost any CPU time, except when using the encrypted folder (and maybe when logging in, but I don't think it's measurable).
You can find out the encryption speed e.g. by using
OpenSSL or whatever). On a 4-core CPU supporting AES-NI it's possible to encrypt 3 GB/s, which is far more than the disk can provide. Without knowing much about your CPU I'd guess it can encrypt about 150 MB/s, which might be a problem for a disk-intensive application if everything was encrypted. Encrypting one folder should be fine, anyway.