I have had issues for a while with upload speed to our samba server. I was able to download at ~70MB/s, but upload at only 7MB/s. From memory, I fixed the download speed by setting a bunch of settings, but uploads remained slow.
Tracing the issue led me to do a strace on the smbd process, which showed 1460 bytes being written every 0.2ms, with the read() and write() system calls using all the time. The end solution was to set the following option in /setc/samba/smb.conf
socket options = TCP_NODELAY