according to How can we make the lantiq xrx200 devices faster
the latest master snapshots have a bunch of patches that will improve the speed of a HH5. Give it a go?
https://git.openwrt.org/?p=openwrt/openwrt.git;a=commit;h=7e484b902d269d8fd22afeccb765ccc2cd8efa19
lantiq: backport latest upstream patches
This patch includes a series of performance improvements. All patches
were accepted and should land in 5.17.
NAT Performance results on BT Home Hub 5A (kernel 5.10.89, mtu 1500):
Down Up
Before 539 Mbps 599 Mbps
After 624 Mbps 695 Mbps