Check if your power plan is configured correctly. It should be in high-performance mode. In control panel, search for power options. Once you're there, select "change plan settings" and find "processor power management" and select "maximum processor state" to 100%. After you do this, restart your system and see if it has fixed the problem. I know you're saying it works the way it's supposed to after a reboot anyway so you might have to shut it down after this and then boot it up to see if it is working properly.
I encountered a strange problem recently with my laptop. If I boot the laptop for the first time, CPU runs approximately on 1GHz, its supposed to run at 2,7 Ghz at maximum. However, if i restart laptop, everything runs perfectly. How can I solve this problem?