The several crossings on the Windsor Lines (North Sheen, Egham, etc) are an absolute cap on capacity because although you could get a few more per hour on the rail side, the road congestion on extremely busy roads (including pedestrians), plus the current high frequency, leads to traffic being backed up all day long. Development long ago right up to the four corners of the crossing prevents any bridge/underpass solution. This is what did in the Heathrow Airtrack scheme, when it was realised how that would make all the crossings untenable local political campaigns reached right up into Parliament.
One issue, from me knowing at least North Sheen for a very long time, is each generation of resignalling has increased road closure times. Before Feltham power box opened (1970?) the local gate box staff were past masters at minimising any delay, there were significant complaints back then when that resignalling took place. The more recent change has again led to further increased closure time per train, it is as if the signalling designers just give this aspect insufficient consideration because it is not a "railway side" issue.