Is that from the timetable? As the 81 is Timetabled to take just shy of an hour, so if there are traffic jams affecting the 81 and making it one and a half hours, then they are likely to affect the 75 too (I know they don't go the exact same way but traffic in the vicinity can affect most local roads).You could consider:
Bus 75 Slough - Heathrow (prices Slough Areas 1 & 2 £24/week)
Picadilly line Heathrow - Hounslow Central (Zones 4-6 Travelcard £25.20/week)
Totalling a rather pricey £49.20/week and still taking an hour
The advantage of this route is that outbound it doesn't matter if you miss a tube train as they're so frequent.
In terms of a route which avoids stage carriages, the only thing I can think of is Great Western to Ealing Bdy, the District line to Acton Town and the Piccadilly line from there.
In this a case (like Epsom or Redhill) where a zones 1-6 travelcard is used for a much longer journey as London Buses operate beyond the Travelcard boundary?
Yes, pretty much all TfL bus routes that go outside the GLA boundary are still on the same fare (i.e. 1.40 a go with Oyster PAYG) / bus pass or travelcard deal as bus journeys wholly within GLA. Therefore, if the journey is solely Hounslow - Slough, a TfL bus pass / Oyster is fine, or a travelcard valid in any zone/s.
Travelcard / Oyster is however not valid on FGW trains west of Hayes & Harlington unless the bit outside zone 6 is paid for separately, and they are not valid on First Berkshire service 75 outside London (I'm not sure they are inside London either - although there is the free travel zone round Heathrow)
Someone has mentioned Heathrow Connect - Last time I looked, that's not Oyster / Travelcard-able from Hayes & Harlington to Heathrow.
TfL Journey Planner suggests most of the alternatives, i.e.
75/6 to Heathrow then piccadilly line
FGW to Hayes & Harlington then H98
FGW to Southall then bus 120
are all going to take about an hour.
It does suggest the Slough - FGW - Ealing Broadway - Acton Town - Hounslow option as taking slightly less than an hour, but don't know how many trains an hour provide good connections here. It's the least likely to be affected by traffic jams, but potentially the most costly option, as you'd need a Slough to zone 3 travelcard for it.
Yes, pretty much all TfL bus routes that go outside the GLA boundary are still on the same fare (i.e. 1.40 a go with Oyster PAYG) / bus pass or travelcard deal as bus journeys wholly within GLA. Therefore, if the journey is solely Hounslow - Slough, a TfL bus pass / Oyster is fine, or a travelcard valid in any zone/s.
Travelcard / Oyster is however not valid on FGW trains west of Hayes & Harlington unless the bit outside zone 6 is paid for separately, and they are not valid on First Berkshire service 75 outside London (I'm not sure they are inside London either - although there is the free travel zone round Heathrow)