7-Zip is fast and convenient. My go-to tool for this purpose for small jobs these days. It has AES-256 encryption, so it is secure.
I am not sure that it would be as convenient with 1.5GB folders though. Is this for online or offline storage?
Do any of your apps need un-encrypted access to these files? 7-Zip is more for archiving data securely, and you would need to enter the password each time you accessed the folder.
If you need direct access to these files daily I would probably recommend BitLocker, but that is only 'free' with Win 8.1 or 10 Pro. You can easily set up a protected disk that automatically unlock when you sign in to Windows. It is a $99 upgrade from Home editions of Windows, but depending on your needs, that my be worthwhile. I have it and use the full disk encryption and love it. Also have it on a couple of external drives.