I have the exact same issue with my laptop. I still cant find a fix for it but heres what I found out
maybe its the BIOS update, ill do a system restore and see what happens. I don't remember if this was an issue before
I tried installing an older version of the BIOS and that didn't seem to work. After some research however this problem seems a lot more common than I initially thought if you google "laptop throttling at 0.78ghz"
anyway I found something that works in my case. the problem seems to be coming from something called BC PROCHOT.. They go more in depth about what this does here but the solution is to either open up the back of your laptop, unplug & reconnect the battery, then dust everything out (which works in some cases). Or to install and run an application called ThrottleStop which disables BC PROCHOT.