After the last Build update 14342 that went out to Windows 10 Mobile Insiders (Fast Ring) a little over a week ago, many users complained about very bad battery performance.
Microsoft has promptly addressed this issue by pushing out a cumulative update to the 14342 build.
The new Build update 143342.1004 is now available, it addresses this issue and I can confirm that after a few hours since updating, the battery life on my Lumia 640 is much better now.
Here is Microsoft's statement on this latest build 14342.1004:
UPDATE 5/25: We are releasing Build 14342.1004 today for Windows Insiders in the Fast ring. This build is exactly the same but with some additional fixes including a fix for the battery life issues we were seeing on some devices such as the Lumia 930, 929 and others which was also causing these devices to get hot.
Source: Windows Blog