Modern hardware with AES-NI can do AES encryption at the speed inwell in excess of 1-2 GB/s. By comparison, your hard disk probably can only write at 50MB/s-100MB/s or SSD about 500MB/s-1GB/s. In other words, with properly implemented hardware accelerated encryption, it's unlikely that encryption speed is your bottleneck here.
If you have a huge file (tens of gigabytes or larger), and you want to be able to edit parts of the file without reencrypting/rewriting the entire file, that's a totally different problem. To be able to do this efficiently and securely, you need to understand about block cipher encryption modes. You'd want to have your encryption system to use encryption mode that supports random access securely, such as XTS mode. Note that encryption mode that supports efficient random access are often considered by some people to be not as strong as serial access encryption mode. You should research and consider if the supposed weaknesses of random access encryption is of concern to your particular scenario.
The easiest way you can use encryption mode that supports random access efficiently, is to mount the encrypted file as a block device/disk using full disk encryption software, like TrueCrypt/VeraCrypt, LUKS, or BitLocker.