Investigate performance issue for datapoints with short time periods

[glatterf42]

@Wegatriespython reported that some tests he developed for addressing an issue in message_ix ran noticeably slower on ixmp4 than on JDBC, in particular the ones using a 1-year time period. This issue is to keep track of this; we should

• investigate if this issue still persists and
• address it if that's the case.