T21P_E2 firmware 84.0.125 slow with VLAN

[GwenH]

At one customer we find the following network-latency problem...
Does anyone have a suggestion for a fix or cause?

Yealink T21P_E2 phones respond slow with firmware 84.0.125
ping-times are above 100ms (close to 200ms)
with version 81.0.110 performance is good, ping-times are below 20ms

All phones are directly connected to a switch and have VLAN configured through provisioning.
- the phones running firmware 81 have no network-latency.
- the phones running firmware 84 have a lot of network-latency.
downgrading the phone to firmware 81 solves the network-latency.
We can reproduce this with firmware upgrade/downgrade, without changing the physical network.

Currently we have downgraded all their (100+) T21P_E2 phones to v81,
but we would like to upgrade them to the latest firmware sometime soon.

At a different customer without VLAN we see no problems with firmware 84.