dont forget hdd seek time on a huge raw file is normal for some lag.. chkdsk /f the disks.. check them for bad sectors just in case. got a good SSD? in 2010 bought a GtS 250 1gb OC and if i woulda had extra $35 i would have got the 260 and my games would have played much better... the 740 is not top of line but shall do major circles around your old card.
defrag alone wont fix bad heads reading slow.. but should help. new hdd new install should say if its vid card or hdd... and its needed every 3 to 5 yrs average hdd life.
i actually believe 740 is a bit weak.. but even with better video card the full hd preview is rendering heavy on it. i believe is somewhat normal issue.