The problem isn't the large number of addresses... it's the fact that large numbers were used for the "address_index" value in the derivation path.
ie. something like: m/44'/0'/0'/0/1000000000
Obviously, as Electrum works sequentially from 0, getting up to the "billions" value is going to take a while (and something it wasn't really designed to do).
You can use iancoleman's BIP39 tool to generate the keys of a specific index range like 1,000,000+
...
Now to generate the keys of the specific index, type the desired index in "starting from index" textbox
and the number of addresses/keys in the "Show" text box, then click "more rows".
Hadn't noticed that Ian Coleman's tool had that "starting index" feature! brilliant stuff...