WB from East Putney to Southfields

With Southfields being my local station, I use the District line all the time. This week, I've noticed that trains have been running slower than normal from East Putney to Southfields. The speed is normal when leaving the WB platform at East Putney, until East Putney tunnel, where I have noticed trains are running slower than normal. After exiting the tunnel, trains seem to slow down as if there is a red signal ahead (I don't even think there are any signals between EP and SF, (yes I know this contradicts my opening sentence)).

Is there any reason for this? This has been the case everyday this week on the trains I have been on.