The default keypool is set to 1000. What does it mean exactly? That I have a pool of 1000 keys/address to use?
If you have a HD wallet (check the icon on the bottom-right corner in Bitcoin Core), it doesn't matter.
If you've created your wallet in an older version of Bitcoin Core, it's not an HD wallet. If that's the case, your keypool is important for backups: your backup has the same 1000 keys, which means you only need to make a new backup after using many different new addresses.
If you would limit the keypool size to 20, you'll quickly run out of addresses, and you need to make new backups very often. That's the reason the keypool was increased from 100 to 1000.
I highly doubt it will load faster with a smaller keypool, other factors ("Loading block index") take much longer.