Tools used in enterprise level to transfer build artifacts from CI server to live server
Try LFTP that has commands to mirror or parallel copy. LFTP can run over many protocols.

The likely problem you have is that you are transfering a lot of data from a build service to a single vm over the Internet. Enterprises use the same tools and protocols but they pay for more bandwidth else ensure that their builds and deploy happens within the same cloud provider. They then deploy to many servers in parallel using standard protocols like SCP with tools like Ansible. The networks within a single cloud provider (Azure, AWS, GCP) are prioritised. This means of you want fat deployment bandwidth, and to run on Digital Ocean, you should build on Digital Ocean. 

It may be the case that a very large amount of the 50k of files dont change on each build. So what you really want is a tool that looks at what files are already deployed. We can expect such tools to use file name, timestamps, file sizes and checksums to check whether a file needs uploading. If your build is stateless and creates 1G of fresh files each time their timestamps are going to be newer that the identical files already deployed. LFTP has an "ignore time stamp" option to avoid transfers of newer but identical files. See this question and answer https://serverfault.com/a/311670