This issue was solved ourself.
We modified /app/middleware/request_throttle.rb with below settings.

[original settings]
:
{
maximum: 800,
hwm: 600,
outflow: 10,
up_front_cost: 50,
}.each do |(setting, default)|
:

[modified setting]
:
{
maximum: 3000,
hwm: 2500,
outflow: 10,
up_front_cost: 50,
}.each do |(setting, default)|
: