MBP getting slightly slow

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Have a look at Activity Monitor (Applications / Utilities /) and select All Processes and sort by CPU to see what the culprit may be.
Also check the "System Memory" tab to see what your "Page ins:", "Page outs:" and "Swap used:" are.

image below uses sorting by CPU as an example

Further reading:

Mac OS X: Reading system memory usage in Activity Monitor
Making The Most of Activity Monitor​

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PS: There is no need for any tool to "spring clean" your Mac. You may use Onyx (free) to run some of the scripts, but Mac OS X usually takes good care of itself.

Have you made sure, you have enough free storage capacity, around 10 to 20 % of the HDD's overall capacity?

And in what ways does the MBP feel slower?
Maybe we can help, or maybe you can find answers via MRoogle?