@Andrew Van Wagner posted:
The financial sector’s “salaries and bonuses” attract “many of our country’s best young mathematicians and physicists”—these talented graduates’ education “has been paid for mostly by either government funds or university endowments”, but activity related to what securities prices are doing “within a nanosecond time frame” adds “little to the financial system’s ability to perform any of its economic functions”.
I don't understand the connection between the first and the second part of your sentence, but I'll focus on what comes after "but." I'd say the activity because it is restricted by a reduced relative clause (that is related to ...) and the prices of securities. Plural nouns are not good attributive modifiers.