Extraído de this Ask Ubuntu pergunta :
Dropbox computes a hash of all files that it's going to sync, it uses it both to determine if they changed and to avoid uploading content that is already in their cloud (another customer has the same file). Calculating hashes takes CPU power. It's usually noticeable at login.
Parece que a única maneira de corrigir isso é reduzir o tamanho da sua pasta compartilhada do Dropbox.