I have had my Desire HD for one week now, and for the first 6 days I was very worried about the speed and the battery life. It was incredibly slow, and would take several seconds for anything to load (even the contacts and messages, I would type a message and no text would appear on the screen for about 10 seconds) and the battery would last about 6 hours (no wifi, gps, 3g turned on, brightness at 30%).
I decided I didnt want the friend stream on anymore, so I removed it from the home screen, and now the phone is lightning fast, and currently the phone has been unplugged for 8 hours and still has about 92% battery (wifi on, browsing internet, sending sms).
Has anyone else found the friend stream to use a lot of power and battery?
(I also removed the advanced task killer as I decided it was fairly useless after reading a lot of posts about it)
It's a known fact that some of the HTC Sense features drink battery faster than an athelete drinks energy drinks. So tbh that was no surprise

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As for using Wifi, that saves a crap load of battery compared to 3g, so using that when you can is a battery saver. Doesn't surprise me again it only drains 1% an hour.
However, at the writing moment I am conducting a test about task-killer apps.
My results are quite interesting (although for me as an electrical engineer, is not surprising). I uninstalled my task-killer app, and rebooted my phone 2 nights ago. Then I had a 30 min long call, and went to bed after starting a whole butt load of apps (facebook, twitter, gmail, people, messages, playing 5 min on angry birds, market, google search, browser, etc.). When I woke up the next morning (and I had the call after fully charging) my phone still had 88% battery left. I was amazed and thought this might actually be true, but after thinking about it, I reconned what I'm gonna tell next. As you use the phone over a duration of time, the RAM starts filling up (the reboot wipped the RAM). As explained in another post, RAM will constantly pull battery power to keep the RAM up (RAM is not like ROM, which is magnetized information, thus not requiring a constant pull of battery like RAM does). This resulted in that as the day progressed, my phone was pulling more and more juice. By the evening at 6 (that is about 20 hours after I unplugged it) it had 45 % left. By the time I went to bed it had only 30% (this was at around 2230). When I woke up the next morning I had to charge it, as it was on about 13% left (this was about 0800).
During this time, I have had 3g on, no wifi, active sync on, always-on internet off, been using the phone slightly frequently (i.e. some facebook, messages, 30 min of music, but no extreme use, just normal, just like I would before I decided to do this test). However, when I checked the battery usage, Android OS (description: Battery used by applications running... I'm guessing the RAM) and Display were on a tied 35% usage. Up time was about 35 hours (same as time since unplugged, as I unplugged it during the boot), but Awake time was "only" 3 hours and 35 mins.
So by not having a task-killer app, my phone lasted only ca. 35 hours, after a fresh reboot. When using a task-killer app, my phone lasted 40 hours+, and still had about 20% juice left when I chose to charge it. And I didn't even reboot. In a few days I will reinstall my task-killer app, and reboot my phone and write Up time, awake time and time since unplugged and all other stats that I can think of.
SO, in conclusion, I can say that yes, to some extent the task-killer app is not needed, BUT it requires that you reboot your phone almost daily, otherwise your RAM will eat up your phone battery.
On another point, I will admitt I noticed a slight improvement on efficiency/performance, but it varied (I'm guessing this is dependent on background CPU usage).
I'll start writing my comprehensive battery guide soon, and post it here
PS! The task-killer app I used before this test, is known as one of the best on the market, "Advanced Task Killer Free" by ReChild. It does not have any bad bugs or drains power like some people claim all task-killer apps do. It is off course important to chose a good task-killer app, or it won't work to its purpose. It's like saying "Oh, all cars use too much gasoline!" It's all about what kind of car and engine you get, that determines how much gasoline it uses. Same applies to task-killer apps
PS2! I found the info from my network counter about the data downloaded during my experiement (that is still going on). The amount of data I downloaded over 3g (no wifi used, no task-killer) with the 3.5 hours awake time was about 10 MB, meaning I did not pull much power on that.