It is hard to make it readable for fast frequencies. And even if you do it well, there will be a frequency limit for every user. So you will either have to limit/funnel the speed, or accept that all tweets are not going to be read.
Instead, an idea could be to divide them in two parts:
- A flowing view for visual effects, with mini versions of the tweets, only to communicate the frequency, and
- A readable view with all text and details, making one selected tweet readable at a time.
I imagine the readable view as a callout from the flowing view. Imagine a map or 3D globe (kind of like this), showing mini versions everywhere on the spinning earth, and has a big "readable window" that presents tweets one at a time, automatically, long enough for users to read it. You do not need a map or globe to do this, a normal list would also work:
download bmml source – Wireframes created with Balsamiq Mockups
The trick is to make it obvious that the readable view "rotates" more or less randomly and that you cannot expect it to display everything if the frequency is high. A user could also select a mini version of the tweet from the flow, to read it, as a feature.
If you choose to let tweets be selected and fade out, make sure they are delayed long enough for a user to finish reading after it faded out, or is easily reselectable to load back into the readable window again, in case the user did not finish reading it.