How I fixed rubberbanding and slowdown when taking a portal.
I just purchased Titan Quest Gold on the steam sale and played for a while but noticed performance issues. The cause seems to be hard drive trashing as Titan Quest seems to write to it constantly. Particularly to the save folder in Documents/My Games/Titan Quest - Immortal Throne/.
The steps I took:
1. Installed "Dataram RAMDisk", google it
2. Created a unformated 512mb ramdisk
3. Formated it as ntfs with windows Disk Management, It got the drive letter E:\
4. Moved the save folder C:\Users\YOURUSERNAME\Documents\My Games\Titan Quest - Immortal Throne\ to E:\Titan Quest - Immortal Throne
5. Created a symbolic link with this command (requires a admin command prompt):
mklink /d "C:\Users\YOURUSERNAME\Documents\My Games\Titan Quest - Immortal Throne" "E:\Titan Quest - Immortal Throne"
6. Played the game with no slowdown.
If you do this you will need to keep your save folder backed up as if your pc crashes that ramdisk and everything it contains will be lost.
This worked for me, I don't guarantee it will work for you and if you screw anything up blame yourself.
I fixed a rubberbanding issue in a very similar way to the OP - I had the game installed on a regular hard drive, but my OS (and "documents" folders, hence save games) was on a separate SSD. Every time I picked up an item the game would hang for a second or so - a really annoying problem. I tried a bunch of fixes, before this one.
Rather than use RAMdisk, I simply moved the save folder onto the same drive as the game and made a symbolic link, as described above. Hey presto - no more hangs.