I bet the files were cached and reads took the same amount of time. The slowdown would be due to writes. tmpfs is allowed to lose the disk contents on reboot, filesystems aren't.
I can write 1G of data to tmpfs, read it badk, and delete it (a typical scientific profile), without ever expecting it to hit rust. I'd be very VERY disappointed in any filesystem that allowed its write buffers to get that far behind.