Constant SMC resets required

It is good you mentioned the Check Battery Warning since that is your problem.

Mac will cut down the CPU speed to 1/2 if the battery is not up to snuff.

Your SMC reset will ratify the issue for a while, but since the battery is on the verge, it will eventually trigger the cutting the CPU Speed/Power consumption to 1/2 of normal. You see that as computer slowdown.

If you want to verify my claim there is a CPU test made by Intel to check the CPU and do that during the slow times as comparison to when it is fast.

Download it, and yes you can keep it :)

Here is a sample what it looks like on my MBA.

If you fix the battery issue and still have performance problems, you could try the solution I found. I had a MacBook Pro that ran dog slow and the problem didn't go away after a reformat and reinstall and a battery replacement. It stemmed from a runaway kernel_task process that you could see in Activity Monitor was using 150% CPU.

After several bouts with the machine over several months, I finally properly tuned my google-fu and found some answers at http://www.rdoxenham.com/?p=259

If your kernel_task is doing the same thing, try this...

Effectively, you just remove your "Model Identifier" .plist file from a package within a package at:

/System/Library/Extensions/IOPlatformPluginFamily.kext/Contents/PlugIns/ACPI_SMC_PlatformPlugin.kext/Contents/Resources/

• You can find the model of your computer: Apple Menu -> About This Mac -> More Info... -> System Report

(or by running system_profiler -detailLevel mini | grep "Model Identifier:" in terminal)

• You can enter a package's content by right clicking it. You'll have to do this twice to get to where you're going.

Keep in mind that you may at this point need to be very careful about making sure you don't overheat your laptop by somehow defeating its ventilation or heat dissipation.

If it doesn't fix your problem, just replace the model.plist.