why do you think there are many SHA256 operations involved?That is what is required to calculate a longer chain that stands a chance of being accepted as legitimate.
The better chain needs to almost mirror the honest one in terms of certain properties.
The retargeting algo in Nxt plays an important role in this.
how would a large hashrate benefit an attacker?See above.
We are talking about POS, right? and specifically you are talking about NXTs implementation, right?
The security of all POS coins is based on the premise that getting 75% of stake is hard, and it's not based on any brute force calculation of SHA256 hashes. Please explain exactly
how would a large hashrate benefit an attacker?No special hashrate (more than the number of accounts per second), is required to create the main chain, and also not any special hashrate would be required to create an attacking chain. You only need the staking power.
Regardless of the baseTarget ajustments (which I seen in the Java code for NXT v1.3.3, not in any documentation because reading NXT docs is terrible, you never know what is actually implemented and what not), if you have more coins than those that were at stake then you can rewrite up to 720 blocks. No need for much hashrate. This applies to all POS implementations that I have seen.