No that is not "normal", but it is also not that uncommon that at higher Bandwidth plans a router's CPU simple has issues doing the demanding traffic shaping work.
You could try the following:
- log into your router via ssh and execute the following command: "top -d 1" (note to stop top you need ot press control-c)
you will see something like:
Mem: 39140K used, 20800K free, 620K shrd, 5032K buff, 9012K cached
CPU: 0% usr 1% sys 0% nic 97% idle 0% io 0% irq 0% sirq
The second row is especially interesting, especially the idle column (how much CPU cycles are "left over" once this approaches zero it is a sure sign, that your CPU runs out of steam, so to speak) and the sirq column (soft interupts, most of the heavy lifting of traffic shaping and wifi happens in sirq "context", so idle going very low and sirq going very hight typically indicates that the load exceeds the router's capabilities).
- start a demanding speedtest, ideally a bidirectional test (maybe by starting two "normal" speedtests staggered in a way that the first is done with the downloadtest, when the second starts so one tests download overlaps with the other's upload).
2.a) monitor what is happening with the top -d 1 output...
A few questions, could you please post the output of the following commands:
cat /etc/config/sqm
tc -s qdisc #(before a speedtest)
tc -s qdisc #(after a speedtest)
cat /etc/os-release
cat /proc/cpuinfo
Oh, and could you post a link to a dslreports speedtest (as shown in https://forum.openwrt.org/t/sqm-qos-recommended-settings-for-the-dslreports-speedtest-bufferbloat-testing/2803/19)