Kernel_task is using 90% CPU, what could be the cause

I understand your concerns (justified or not), you love your Mac and want to keep it alive as long as possible.

Here are some home remedy (Do it Yourself)-- (also available where you live).

If you want to keep your Mac cooler during HOT games, go to pharmacy and buy what they call a gel-pack (gel balení). Freeze it!

Then put it under your Mac with a towel in between (to prevent freezing).

That will provide additional cooling for few hours. Have second gel-pack if you want to continue gaming for 24 hours a day :)

As for your temperature reports, so far they look normal based on this example.

Throttling

This test wouldn't be complete if we didn't address the subject of throttling. Prime 95 and Furmark can extract the maximum performance out of the hardware. When both programs are run simultaneously, the core frequency drops (caused by Prime 95) to 1.2 Ghz. Running Prime 95 alone, we measured temperatures between 92 and 104 degrees Celsius (197.6 and 219.2 degrees Fahrenheit), and the word "Throttling" flashes repeatedly in HWiNFO. Furmark on its own cycles through all available Turbo Boost steps. After about one hour, the temperature settled at a fairly consistent 85 degrees Celsius (185 degrees Fahrenheit) with the fan system running. We never measured above 100 degrees Celsius (212 degrees Fahrenheit) with Furmark. This limitation might be due to the power adapter: in the above scenario, the system would require 86.6 watts - but the power adapter can only supply a maximum of 85 watts. Immediately after running our different tests, we subjected the system to 3DMark 06 and CineBench R11.5. The results were identical to after a cold boot.

The fastest way to quick check is to use the surface temperatures as shown here.

As far as I am concerned kernel_task was always trying to tell me that the computer is running hot. It seems to be a strange countermeasure which occupies the CPU with a low energy No-Op task which prevents it from doing "hotter" things. So you should be looking for that "other" process eating up the rest of your CPU time.

However I noticed very high CPU usage of Safari ever since I update do Yosemite 10.10.4. The browser visibly (in Activity Monitor) starts several processes now and two of them (Safari Networking and nsurlstoraged) routinely take up a lot of CPU, especially Safari Networking often takes more than 50% of one CPU. It seems to be less constant now, so maybe there is some "trickle down" effect on this one.