PERF: investigate performance impact of iNaTs on tz-aware operations
#### Problem description

Placeholder issue for use of `np.searchsorted` in `pd._libs.tslibs.conversion.tz_convert_dst()` introduced in #22181 and modified in #24491. The latter indicates that the `iNaT` optimization comes at high cost for notnull data, but it's unclear at what fraction null should we attempt to optimize by skipping the `np.searchsorted()` call entirely.
