Slow functions

[NickCH-K]

The following pmdplyr commands are too slow, to the point even where the examples take >5s, and would ideally be faster:

• tlag when .d=0 (slow because it currently performs a join)
• panel_fill
• panel_locf
• mutate_cascade

Slow speed in many cases is driven by the need to work when .i and .t do not uniquely identify observations, so one possible means of speeding up in those cases is to check upon call whether they do uniquely identify data, and divert to a quick method if they do. tlag already has an option to do this the user can select. But ideally all cases will be sped up.

[NickCH-K]

tlag (both .d=0 and otherwise) considerably sped up in 108e3c6

[NickCH-K]

panel_fill improved in 58bb82b but could stand to be improved more

[NickCH-K]

panel_locf improved massively in d255fad and now at a good place.

[NickCH-K]

mutate_cascade improved modestly in 1aaae84. Not sure how much faster it can get without a major restructuring.