I also got the red box in the corner, I'm assuming it was the framerate. The vehicle actually ran a little better than the standard game on my machine, but that just might be because when I loaded the Beta, it wiped all of the stuff I built, all the dead zombie, about 1343 I think, from around my base, and everything I built. Found the car and was able to drive around for about 10 minutes with no lag issues in gameplay. I did notice that I was having trouble putting large objects into the Postal Truck, I was able to get half a metal shelving unit into the truck after loading nails into it first, but could not put in the second half of the shelving unit. Even if I added a smaller item to the empty side first.
My machine specs are as follows.
Windows 10 64 bit with all the latest patches and Norton Security Running.
CPU
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Model : Intel(R) Core(TM) i5-6400 CPU @ 2.70GHz
Logical cores : 4
CPUID : GenuineIntel family 6, model 5E, stepping 3
Tjmax : 100°C
GPU1
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Display device : GeForce GTX 950 on GM206-A GPU
Display driver : 376.53
BIOS : 84.06.2f.00.7d
GUID : VEN_10DE&DEV_1402&SUBSYS_36C31458&REV_A1&BUS_1&DEV_0&FN_0
Registry key : \Registry\Machine\System\CurrentControlSet\Control\Video\{152F1C2C-4A15-4B7D-8F67-28021F72ABF7}\0000
VDDC controller : Display driver
8GB RAM
Hope this helps.