"kernel_task"?

It is the core routines for OS X that are used for all the software running on your computer. It is not a file but a process represented by all the sub-routines called by all the software that uses kernel tasks. It does not run slow and it will use a lot of space. Don't worry about it. If your computer is running slow that is not the reason unless the kernel_task has crashed.

Ways to help make a slow Mac faster

17 Reasons Why Your Mac Runs Slower Than it Should

Slow Mac Performance? This Article Solves It!

Fix slow start-ups in OS X | MacFixIt - CNET Reviews

How to fix slow shutdown and startup times. | MacTip.net

6 Easy Tips to Speed Up OS X Yosemite on Your Mac

Avoid using third-party software that claims to clean up your computer. Usually this software does more bad than good. Furthermore, you don't need it. Note that all computers will become slower over time even under normal use. Experienced users typically erase the hard drive and do a clean install from scratch at least once a year or whenever installing a major OS upgrade. Of course doing so also means you must maintain regular and multiple backups.

Add more RAM or cut back on the number of concurrently running applications and utilities. Remove unnecessary software such as anti-malware and software that promises to clean your Mac. Check for runaway processes: Runaway applications can shorten battery runtime, affect performance, and increase heat and fan activity. Also, visit The XLab FAQs and read the FAQ on dealing with The Spinning Beach Ball of Death.

The Ultimate Fix

Backup everything, erase the drive, reinstall OS X, and restore your data from the backup. Reinstall third-party software from original media/scratch.

[Permission to use part or all of the above has been granted by Kappy, exclusively, to theratter.]

so much memory

How much?

Perhaps the amount used by kernel_task is just the normal?

It is.