So during my first session of GTA V which was good I alt'tabbed to check notifications on Facebook and noticed that my pc is really slow as in typing in letters it was really slow as i typed in the letter and it would take at least half a second to show up on screen. Also that in certain parts of the game I noticed my the characters are getting pixelated.
Here are my Specs Intel Xeon 1231v3 Asus H81M-E 2x4 Kingston Blu 8gb sticks R9 270X Asus Top 2gbs Silverstone Strider Plus 600W Fully Modular Cooler Master Hyper 212 Evo Cougar Spike 5SS7 Mini Tower Case Windows 8.1 Pro 64 Bit.
Everyone is experiencing it. I am pretty sure there is no fix for it. Regardless of specs. GTA V tabbed out take quite a hit to the desktop. Even in windowed borderless I experience the same thing. Just tabbing in and out of GTA V is much quicker.
It is known and the fix is usually turn off the "pause when focus lost" option.
EDIT: It was once on this page http://pcgamingwiki.com/wiki/Gta_5 . if it fixes your problem I will edit the Wiki again and add it back, no idea who removed it, maybe it stopped working after an update.