Many people complain the timeline preview not fluently as a normal player.

Why that can play it back so and VEGAS can't?

One thing you forget:
The timeline is building with many events mirroring different clips with different dimension/codec/framerate and bit-rate...from the one to the next one, VEGAS must release as quick as possible the old cache and decode with new caching; once it congests in the last cache, it surely can't build the next new one.

So, if you see repeating dots at Frame: xxx ... you should stop the playing back and restart it.
If your restart follows after 2 or 5 frames the repeating dots, it indicates your HW is poor or VEGAS is not adjusted properly in its F/I-Tab, or the codec-structure is too hard, e.g. my machine can never play back 10bit422HEVC.