Is it possible to sync whole folders in Dropbox app? I currently download file by file a slow process
Sync isn't possible and the "download folder" function is also missing on Android.
You can support this feature request: https://www.dropbox.com/votebox/2133/android-download-upload-folders
As answered here, you can use file explorers such as ES file explorer to achieve this functionality.
I use the dropsync (comp.ttxapps.dropsync) app to keep full synchronization on my big dropbox which I have shared on several computers and several android devices. I've rarely had problems getting large volumes of files per directory trees in my dropbox handled. The only issue I've encountered is when the particular device had a default bandwidth limit applied, which I generally reset to unlimited.