Get the drivers from DELL website http://www.dell.com/support/drivers/us/en/19/Product/inspiron-15r-se-7520 and select Windows 8 64-bit. Performance degredation is normal - "newer isn't always the best", wait for the OS to mature, for drivers to get better etc.
-------------------------
Intel I7 3770K @ 4.5ghz, Cooler Master Hyper 212 Evo, Gigabyte Z77X-UD3H, Gigabyte HD7970 ghz edition, 4x4gb Kingston HyperX Beast 2133mhz, Seasonic Platinum 860, OCZ Vertex 4 128gb, WD Black 1TB, WD Green 3TB+1.5TB, ASUS Xonar Essence ST, ASUS VE278Q, Windows 3.11