Rate my PC? Built about 2-3 years ago

Been having some issues with my CPU running a bit slower and finding it harder to play certain games but was wondering what parts I should consider replacing.

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Processor: Intel(R) Core(TM) i5-6600K CPU @ 3.50GHz Quad-Core

Motherboard: Asus - Z170-A LGA1151

Memory: Crucial-Ballistix Sport 8 Gb (2 x 4 GB) DDR4-2400 Memory

SSD: Samsung 850 Evo-Series 250 GB 2.5"

Video Card: Nvidia Geforce GTX 980 4GB Superclocked ACX 2.0

Windows 10 (64-bit)

Acer 23.6" ED242QR 1920x1080 144Hz

Asus 27" idk what it is, it's junk

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