Balham to West Croydon via Crystal Palace always seems incredibly slow:
20 out of Balham and over the junction, then up to 30 until Streatham Hill depot, then linespeed rises to 50 but you're lucky to get to 35-40 before needing to brake for Streatham Hill.
50 leaving Streatham Hill but you might make 40-45 before braking for the 30 at Leigham Jct which continues along the spur and over West Norwood Jct before dropping to 20 through West Norwood.
Just beyond West Norwood linespeed increases to 35, then 50 but again you're lucky to get to 40 before braking for Gipsy Hill. Linespeed is 50 into Crystal Palace tunnel but we're not supposed to go much above 30-35 in case of being wrong routed in the tunnel.
Then it's 15 over the junction and through the platform, 25 once clear of the platform, then linespeed rises to 50, but if you're going to Norwood Jct the signal at Bromley Jct will be at red so you can't take too much power, maybe 35-40 tops.
20 over Bromley Jct, 30 down the spur but linespeed drops down to 20 at the bottom so there's no point taking power. 20 pretty much all the way into Norwood Jct, especially as the starter will be at red if routed into P6 (approach control again).
25 out of Norwood for the crossover onto the down Wallington, then maybe up to 35 before needing to brake for the 20 speed on the overbridge, which continues for what seems like ever over Gloucester Rd Jct before rising to 60 for a short distance before dropping again to 45 on the bend into West Croydon - but it's very common to be held at a red outside W Croydon if there's a LOROL terminating in the platform so the 60 speed is irrelevant.
Average speed: maybe 30 mph?