Doing some quick research it looks like
* The Picadilly line takes 41 minuites from south kensington to heathrow terminals 2 and 3
* The Lizzie takes around 31 minuites from paddingto to heathrow terminals 2 and 3
* Heathrow express takes about 17 minuites from paddingto to heathrow terminals 2 and 3
*IF* your origin can reach heathrow in fewer changes by using the Lizzie then that is likely to compensate for the slower train, but having looked at the maps, my feeling is that probably applies to less than half of potential origins.
Out of the 11 major tube lines
* 4 go to paddington.(Circle, District, H&C bakerloo)
* 4 have one or more interchanges with the lizzie in central London but do not go to paddington (Central, Jubilee, Metropolitan, Northern)
* 2 have no interchange with the lizzie at all
* 1 only meets the lizzie at heathrow (Picadilly)
If we look at London terminals.
* 1 has heathrow express service (paddington)
* 2 are on the lizzie (liverpool street/moorgate)
* 3 are not on the lizzie but have direct tube services to paddington (Marlybone, Euston, St pancras, Kings Cross, Cannon Street, Charing Cross, Waterloo, Victoria)
* 1 requires a change to reach paddington, but can reach the lizzie with one tube or thameslink hop (London bridge)
So I would argue that for *most* origins Heathrow express is still the fastest rail option, whether it's worth the extra cost is another matter.