The trackbed through the Horsham station area has recently had a lot of geological and “wet bed” issues. This may well account for any changes to lower speeds. The junctions also get a hammering with very diverse movements being accommodated, although marginally less now that TL run fixed formations which do not have different portions working in and out of sidings. (When it was the centre of Southern’s universe, a couple of years ago, the stock workings really were a sight to behold!) It’s not just the bits north of the station, even though those have caused a lot of the headaches.