It seems that the latest version of Whatsapp Desktop loads historical data from your mobile Phone, not from the cloud (server).

That is why, it is slow.

To speed it up, make sure both the phone and PC are connected to the same WiFi.

Otherwise, the loading may get stuck forever.

However, that doesn't warrant a service guarantee.

If it fails, leverage another method - install a screen recording app in your phone and then use the Whatsapp app in the phone to locate the old chats or messages by searching, scroll down the messages to record.