Problem is I dont have enough space (now its about 6 years of blocks) and I need a guidance about pruning ( I use Bitcoin Core wallet)
Keep in mind that pruning still downloads all those 300GB (minus what you already have), hence it's still slow.
Exporting the private key is faster, but then you have to make more security precautions: at least make sure you download electrum from the correct site, verify its signature before install and obviously make sure the computer you are working on is free from malware, since the private key will be exposed (i.e. visible) and whoever has it can spend your funds.
Now back to pruning. I've found a pretty good documentation you may want to read, it's here:
https://coinguides.org/bitcoin-blockchain-pruning/If you want to "shortcut", then locate your exe file and run it with -prune parameter, like
"C:\Users\<username>\AppData\Roaming\Bitcoin\bitcoin-qt.exe" -prune=550
or if you're in that folder
bitcoin-qt.exe -prune=550