I really wanted an Email client for Linux which isn't power hungry but unfortunately last I tried Nylas - it was always top app in `powertop`. An always running app has to be low on power consumption IMO, but because so many apps are being built on top of webkit/electron most of them pretty much suck when it comes to battery usage. Another case in point is slack app. On Linux, it is probably the worst app. :(
Slack is the worse offender on my 2011 MBP. I have to kill it all the time because of runaway resource usage.
Never really compared their resource usage since I'm only in one Slack team 99% of the time and even the official app isn't that horrendous in that use case.