What CPU should I upgrade to (200$ budget, iMac)

I have a mid 2015 IMac and it’s starting to run pretty slow, im already getting a new ssd, so what CPU should I get? If it helps the SSD I’m getting is Crucial BX500 1TB 3D NAND SATA 2.5-Inch Internal SSD

IMAC mid 2015 specs: 3.3 GHz Quad-Core Intel Core i5 24 GB 1600 MHz DDR3 AMD Radeon R9 M290 2 GB