Realized volatility is a long-memory process and so I fitted an ARFIMA(0,d,0) to log(RV15) where RV15 is realized volatility calculated from 15-min bars. I proceeded to examine how changing the bar size from which RV is constructed impacts the $d$ parameter in ARFIMA(0,d,0). Below are the plots. It seems that the parameter $d$ decreases with increasing the bar size (see left figure). Lower $d$ parameter means a "shorter" memory time series (see right figure).
I cannot properly explain why this happens. That is, I cannot sit down and verbalise it to explain it to someone else. And even with my explanation, I'm not sure it is correct.
My explanation is the following. Using higher resolution when calculating RV means more information about intraday trading patterns is summarised in the RV. This increase in information contained in the RV allows the value from 20 days ago to better explain the value today. This also captures patterns that repeat themselves and they increase the correlation whereas the lower resolution does not capture these patterns as well.