[DNSB]

The only time I've—in person—seen slowdown from composite columns was when the calibre install had over 80 custom columns with some rather complex and weird templates. That one was solved by asking what use was made of the columns and disabling the ones for which no use was found.

Even then, the major effect was reducing the startup time by about 4 seconds since the values for fewer composite columns needed to be calculated at startup.