Ok, so here is the output of iperf3. If it's relevant, the server is a VM running Ubuntu 16.04, and the machine I'm transferring to/from is a MacBook Pro running OS X 10.13.1
Here is the VM side with the VM running perf -c:
Connecting to host 192.168.1.37, port 5201
[ 4] local 192.168.1.134 port 37876 connected to 192.168.1.37 port 5201
[ ID] Interval Transfer Bandwidth Retr Cwnd
[ 4] 0.00-1.00 sec 28.9 MBytes 242 Mbits/sec 0 1.03 MBytes
[ 4] 1.00-2.00 sec 26.2 MBytes 220 Mbits/sec 0 1.03 MBytes
[ 4] 2.00-3.00 sec 21.2 MBytes 178 Mbits/sec 0 1.03 MBytes
[ 4] 3.00-4.00 sec 27.5 MBytes 231 Mbits/sec 0 1.03 MBytes
[ 4] 4.00-5.00 sec 26.2 MBytes 220 Mbits/sec 0 1.03 MBytes
[ 4] 5.00-6.00 sec 27.5 MBytes 231 Mbits/sec 0 1.03 MBytes
[ 4] 6.00-7.00 sec 27.5 MBytes 231 Mbits/sec 0 1.03 MBytes
[ 4] 7.00-8.00 sec 27.5 MBytes 230 Mbits/sec 0 1.03 MBytes
[ 4] 8.00-9.00 sec 22.5 MBytes 189 Mbits/sec 0 1.03 MBytes
[ 4] 9.00-10.00 sec 27.5 MBytes 231 Mbits/sec 0 1.03 MBytes
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval Transfer Bandwidth Retr
[ 4] 0.00-10.00 sec 263 MBytes 220 Mbits/sec 0 sender
[ 4] 0.00-10.00 sec 261 MBytes 219 Mbits/sec receiver
And here is the Macbook (non-VM machine) with it running iperf -s:
Accepted connection from 192.168.1.134, port 37874
[ 5] local 192.168.1.37 port 5201 connected to 192.168.1.134 port 37876
[ ID] Interval Transfer Bandwidth
[ 5] 0.00-1.00 sec 24.9 MBytes 209 Mbits/sec
[ 5] 1.00-2.00 sec 25.8 MBytes 216 Mbits/sec
[ 5] 2.00-3.00 sec 21.4 MBytes 180 Mbits/sec
[ 5] 3.00-4.00 sec 28.4 MBytes 238 Mbits/sec
[ 5] 4.00-5.00 sec 26.2 MBytes 220 Mbits/sec
[ 5] 5.00-6.00 sec 26.7 MBytes 224 Mbits/sec
[ 5] 6.00-7.00 sec 28.3 MBytes 237 Mbits/sec
[ 5] 7.00-8.00 sec 26.5 MBytes 222 Mbits/sec
[ 5] 8.00-9.00 sec 22.8 MBytes 191 Mbits/sec
[ 5] 9.00-10.00 sec 28.0 MBytes 235 Mbits/sec
[ 5] 10.00-10.07 sec 1.88 MBytes 240 Mbits/sec
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval Transfer Bandwidth
[ 5] 0.00-10.07 sec 0.00 Bytes 0.00 bits/sec sender
[ 5] 0.00-10.07 sec 261 MBytes 217 Mbits/sec receive
Here, also, is the reverse, with the VM running iperf -s:
[ 5] local 192.168.1.134 port 5201 connected to 192.168.1.37 port 54295
[ ID] Interval Transfer Bandwidth
[ 5] 0.00-1.00 sec 2.37 MBytes 19.9 Mbits/sec
[ 5] 1.00-2.00 sec 899 KBytes 7.37 Mbits/sec
[ 5] 2.00-3.00 sec 731 KBytes 5.99 Mbits/sec
[ 5] 3.00-4.00 sec 847 KBytes 6.94 Mbits/sec
[ 5] 4.00-5.00 sec 1.34 MBytes 11.2 Mbits/sec
[ 5] 5.00-6.00 sec 618 KBytes 5.06 Mbits/sec
[ 5] 6.00-7.00 sec 1.53 MBytes 12.8 Mbits/sec
[ 5] 7.00-8.00 sec 1.45 MBytes 12.1 Mbits/sec
[ 5] 8.00-9.00 sec 974 KBytes 7.98 Mbits/sec
[ 5] 9.00-10.00 sec 1.37 MBytes 11.5 Mbits/sec
[ 5] 10.00-10.00 sec 0.00 Bytes 0.00 bits/sec
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval Transfer Bandwidth
[ 5] 0.00-10.00 sec 12.2 MBytes 10.2 Mbits/sec sender
[ 5] 0.00-10.00 sec 12.0 MBytes 10.1 Mbits/sec receiver
And the Macbook (non-VM machine) on the LAN running iperf -c:
[ 4] local 192.168.1.37 port 54295 connected to 192.168.1.134 port 5201
[ ID] Interval Transfer Bandwidth
[ 4] 0.00-1.00 sec 2.50 MBytes 20.9 Mbits/sec
[ 4] 1.00-2.00 sec 898 KBytes 7.35 Mbits/sec
[ 4] 2.00-3.00 sec 732 KBytes 6.00 Mbits/sec
[ 4] 3.00-4.00 sec 843 KBytes 6.91 Mbits/sec
[ 4] 4.00-5.00 sec 1.34 MBytes 11.3 Mbits/sec
[ 4] 5.00-6.00 sec 619 KBytes 5.07 Mbits/sec
[ 4] 6.00-7.00 sec 1.53 MBytes 12.8 Mbits/sec
[ 4] 7.00-8.00 sec 1.45 MBytes 12.1 Mbits/sec
[ 4] 8.00-9.00 sec 974 KBytes 8.00 Mbits/sec
[ 4] 9.00-10.00 sec 1.37 MBytes 11.5 Mbits/sec
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval Transfer Bandwidth
[ 4] 0.00-10.00 sec 12.2 MBytes 10.2 Mbits/sec sender
[ 4] 0.00-10.00 sec 12.0 MBytes 10.1 Mbits/sec receiver
I should also note that I had a server with just a bare metal setup, running the services without any VMs, and I was getting ~15 MB/s transfer rates both ways over wifi.
Thank you so much for any help.
EDIT
I went into the config files at /etc/pve/firewall/ and /etc/pve/nodes/<nodename>/host.fw and manually set the firewall to enable: 0 on the cluster, node, and VM level, restarted the host machine, and now I'm getting transfer speeds 5x higher from MacBook to VM. I had all applicable firewall settings disabled in the GUI at host:8006, but they seemed to have to be explicitly stated in the config files to have any effect. Are there any more firewall settings I can turn off in the config files to improve the speed a bit? It's not amazing, but a 6-7MB/s transfer speed at least makes Proxmox useable.