Personally, I have always thought the slow performance with the bitcoin core client was due to it being behind 2 firewalls with no port forwarding to accept incoming connections from the bitcoin network. I'm not willing to change that for the workstation the bitcoin core client runs on so I didn't see a way to research the slow performance downloading blocks. But I have noticed depending on the day, that the client would have problems accessing peers.
Firewalls are not the usual cause for slow machines, as those only interfere with connection opening. It's often network latency and speed, CPU and disk which weight the client down.