Much more information needed...
what nbn type?
how are you testing?
over wifi or cable?
So FTTC should easily be able to do 100Mbps. I assume 55Mbps is your general max speed, not just in peak hours?
If you ring Optus they should be able to run a speed test from their end to confirm the issue is in the street. After that they should get an NBN guy out to check the issue - OPtus are legally required to give you mostly 85Mbps+ on speed pack 4. @Jeneral__Pain may have some other suggestions to test though.
If you have multiple phone sockets, this mightn’t be helping the situation.
Try plugging your desktop straight into the FTTC NTU and try a speed test. If the speed is still poor, reboot the NTU. If it restores, it means a line condition has caused it to step the speed down to cope with the condition. If the speed does not restore, read on.
if the speed is “slower” 24x7 then it’s more likely to be a fault isolated to the DPU servicing your home. If the speed only drops during “peak hour”, it’s more likely Optus needs to provision more bandwidth in the area. Either way, report this to Optus with everything you’ve tried.