What makes you think that? I don't think that is correct. The Steem Power compounding is perpetual.
I read form a post by dan himself if I remember correctly... I think the interest rate changes
I am bit confused about this. There is one form of compounding that comes from the perpetual creation of 9X Steem Power for every 1 Steem that is minted. Those 9X SP are distributed proportionally to all SP holders. So your SP holdings are always increasing. I don't think it ever ceases nor changes.
Is there also another form of interest paid on SP?
The 9-1 ratio between SP and Steem never changes. I'm pretty sure what he means by the "high interest rate" is the numerical reduction in the rate of effective interest as the supply increases.
Example starting with a supply of one coin:
Add first coin (interest/inflation is 100%)
Add second coin (interest/inflation is 50%)
Add fourth coin (interest/inflation is 33 1/3%)
etc.
Hopefully this explains it correctly:
https://steemit.com/interest/@bacchist/steem-power-interest-is-not-compound-interest#@anonymint/re-bacchist-steem-power-interest-is-not-compound-interest-20160723t033339934z
The problem with this design is that if everyone wants to power up, then the STEEM POWER investors are paying for all the exponential debasement, which is paying for blogging (and mining).
The Bitcoin money supply was debased at 100% per annum only the first year in 2009 and is now around 5%; whereas, Steem plans a perpetual 100% annual minting rate, but much of that is a forward stock split not debasement (approximately 2.8% per annum[1] appears to be the typical rate of debasement of STEEM POWER holders assuming STEEM remains about 10% of the money supply). Forward stock split means the price drop due to money supply increase is compensated with an increase in the number of tokens held.
In order to pay for the blogging without taking it collectively roughly 25% (9/10ths of 2.8% versus 1/10th of 100%) from STEEM POWER investors pockets, will require significant demand to hold STEEM tokens (note ownership changing hands rapidly is still demand as long as the demand doesn't want to power it up).
[1] 50% of 2 of the 4 STEEM created are paid as STEEM POWER for rewards, and that is at a ratio of 1/9th of STEEM POWER money supply per annum.