What are the MCS bit rates on your link? Slow download suggests the C5 receiver is not as good as the 3600, not the transmitter.
RX/TX from main router (archer c5)
26.0 Mbit/s, 20MHz, MCS 3
14.4 Mbit/s, 20MHz, MCS 1, Short GI
19.5 Mbit/s, 20MHz, MCS 2
14.4 Mbit/s, 20MHz, MCS 1, Short GI
40.5 Mbit/s, 40MHz, MCS 2
21.7 Mbit/s, 20MHz, MCS 2, Short GI
14.4 Mbit/s, 20MHz, MCS 1, Short GI
7.2 Mbit/s, 20MHz, MCS 0, Short GI
The WDS client is a GL-AR150 which is 2.4ghz 802.11n