Slow IPIPv6 Tunnel download throughput on CCR2004

I recently purchased a CCR2004-16G-2S+.

My ISP provides a 10G connection with IPv4 over an IPIPv4 tunnel.

With another OpenWRT box and the same configuration, I could achieve around 6Gbps download and 3Gbps upload speeds.
However, with the CCR2004, I’m only getting about 300Mbps download, while upload speeds remain normal, which is weird.
I observed only 1 CPU core is used at ~30% during download tests, 3 CPU cores are used at ~40% during upload tests.

I use the CCR2004 (or the OpenWRT box) to route traffic to the internet for a downstream router, so I do not need any firewall or NAT settings.

Assuming my actual public IP is 1.1.1.1, I’ve set up the router with IP 1.1.1.2/30 and assigned 1.1.1.1/30 to the downstream router with 1.1.1.2 as the gateway. This way, I can access the public internet transparently.

Could anyone help me with it?

Thanks!

Here is my configuration with my IPs masked.

# 2024-11-13 10:43:39 by RouterOS 7.16.1
# software id = R9IL-JP7R
#
# model = CCR2004-16G-2S+
/ipv6 settings
set accept-router-advertisements=yes

/interface bridge
add name=pass
add name=wan

/interface bridge port
add bridge=wan interface=sfp-sfpplus1
add bridge=pass interface=sfp-sfpplus2

/ipv6 dhcp-client
add add-default-route=yes interface=wan pool-name=inet6 pool-prefix-length=60 \
    request=prefix

/ipv6 address
add address=::masked from-pool=inet6 interface=wan

/ipv6 dhcp-server
add address-pool=inet6 interface=pass name=inet6

/interface ipipv6
add local-address=masked mtu=1460 name=ipip6 \
    remote-address=masked

/ip firewall connection tracking
set enabled=no

/ip address
add address=1.1.1.2/30 interface=pass network=1.1.1.0

/ip route
add check-gateway=none disabled=no distance=1 dst-address=0.0.0.0/0 gateway=\
    ipip6 routing-table=main suppress-hw-offload=no

/ip firewall mangle
add action=change-mss chain=forward new-mss=1420 passthrough=no protocol=tcp \
    tcp-flags=syn