Improve speed for multiple file SFTP Upload in R (using RCurl::ftpUpload)

Question

I'm trying to upload all files in a certain upload.dir to an SFTP Server using RCurl::ftpUpload.

Currently therefore I am using a for-loop, which unfortunately takes too much time - I think because it is opening and closing the connection for every file. Is there any way to do this faster?

Here is a code snippet:

files_for_upload <- list.files(upload.dir, 
                               full.names=TRUE, 
                               recursive=TRUE)

files_for_upload2 <- list.files(upload.dir, 
                                full.names=FALSE, 
                                recursive=TRUE)


for (i in 1:length(files_for_upload)) {
  RCurl::ftpUpload(what=files_for_upload[i], 
                   to=paste0("Directory_on_my_server/", files_for_upload2[i]), 
                   userpwd="my_userid:my_password", 
                   .opts=list(ftp.create.missing.dirs=TRUE, forbid.reuse=TRUE)
  )
}

So far I already tried to parallelize the loop with the foreach package:

foreach::foreach(i=1:length(files_for_upload)) %dopar%
  RCurl::ftpUpload(what=files_for_upload[i], 
                   to=paste0("Directory_on_my_server/", files_for_upload2[i]), 
                   userpwd="my_userid:my_password", 
                   .opts=list(ftp.create.missing.dirs=TRUE, forbid.reuse=TRUE)
  )
}

This however gives an error:

task 3 failed - "Failed to connect to My_Server port 22 
  after 21252 ms: Couldn't connect to server"

Is there any solution to speed things up? Like opening a connection and use it for all files? Or maybe upload a zip file and unzip it on the server using R?