It's slower, but not much slower. Heathrow to Central London (Holborn for the British Museum, say) is about 50 minutes on the tube and wait-for-HEX + 15m + walk around Paddington + 12m for about 40 minutes total via Heathrow Express and Tube.

The cheaperER (not in any absolute sense cheap for the value received) HEX tickets are 90 day advance purchase, which is faintly ridiculous. If you have 90 days notice and a burning desire to still overpay, by all means use the HEX.