Is there an alternative to Dropbox which has an smaller CPU overhead Ok, this is not necessarily strictly Mac, however it does affect my Macs differently. 

I have an 5 years old iMac which is ok with Dropbox running all the time, while my brand new MBP (2017) struggle with Dropbox's CPU overhead (struggles is an exaggeration but it does get the fan running at full blast most of the time). I'm a heavy user and have the higher plan, which means there are some files that are constantly being modified, yet I find it extremely annoying to have it happening.

Is there an alternative which uses less resources?
This annoyed me too so I built Rsync Client.  It's like an FTP app but uses rsync, and it only runs when you invoke it, so it's not relentlessly chewing resources. It works with many backend storage service providers, so you control where your data goes.