After some searching on the topic, I found that the socket options line that was recommended in the smb.conf file in previous versions:
socket options = TCP_NODELAY IPTOS_LOWDELAY \
SO_KEEPALIVE SO_RCVBUF=8192 SO_SNDBUF=8192
was actually causing this issue. Anything specified here that is actually required seems to be default with 10.10 and the corresponding kernel, so I just commented this line out, and things were back to a reasonable speed.
I'm guessing that specifying the buffer sizes causes havoc with the current implementation.