I had a viewer ( ask me a question about slow speed between vlans on the USG)
There are a few factors of which can cause this. First off if you have threat management enabled you will see slower speeds from vlan routing as well as to the WAN. Below are the speeds that you will get depending on the model of USG.
Enabling IDS or IPS will affect the maximum throughput on inter-VLAN and egress traffic. USG: 85 Mbps* USG-Pro: 250 Mbps* USG-XG: 1 Gbps* Enabling Smart Queues or DPI on top of IPS/IDS will also incur a further throughput penalty to maximum throughput. UniFi Dream Machine throughput: 850 Mbps* *Values are rough estimates and can vary depending on configuration.
Another factor would be if you have smart queuing turned on. This will limit the data flow between vlans