Quickest would be Underground to Paddington (various routes) then FGW to Slough (should be around 50-55 minutes). ...
Agreed.
I think, if you're a reasonably fast walker, the quickest route between Paddington & Bethnal Green might actually be to
walk between Paddington (national rail) and Lancaster Gate, then direct on the
Central Line to Bethnal Green.
I recommend using the TfL journey planner:
http://www.tfl.gov.uk/
As you can see, Lancaster Gate - Bethnal Green is 18 minutes every 2-3 minutes. This is a very fast, and frequent, route.
If you do not want to walk half a mile, the least walking option would be to use the
footbridge at Paddington and take the
Hammersmith & City Line from Paddington to Mile End, then cross-platform to the
Central Line Mile End to Bethnal Green. This would take approximately 35 minutes with a
potential additional wait of up to 10 minutes at some times, so it could take 45 minutes though it should take less.
A compromise would be to take the
Bakerloo Line (more useful than H&C
if you arrive at the very front of the train in the main train shed at Paddington ([stn]PAD[/stn]) and/or wishing to use the station facilities on the concourse) to Oxford Circus, then the
Central Line. That would take around 30 minutes.