fix: faster picking when slices are made

[henryiii]

This is a followup to #645, and closes #644. Benchmarks:

Before:

---------------------------------------------------------------------------------------------- benchmark 'Pick': 2 tests ----------------------------------------------------------------------------------------------
Name (time in us)               Min                     Max                    Mean                 StdDev                  Median                    IQR            Outliers         OPS            Rounds  Iterations
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
test_pick_only             469.6110 (1.0)       10,963.2400 (1.0)          548.6303 (1.0)         335.7542 (1.0)          496.3605 (1.0)          58.5930 (1.0)         7;153  1,822.7210 (1.0)        1054           1
test_pick_and_slice    752,036.0140 (>1000.0)  794,440.9860 (72.46)    768,945.9046 (>1000.0)  15,980.4104 (47.60)    764,982.2400 (>1000.0)  18,759.4093 (320.16)        2;0      1.3005 (0.00)          5           1
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

After:

--------------------------------------------------------------------------------------- benchmark 'Pick': 2 tests ---------------------------------------------------------------------------------------
Name (time in us)              Min                   Max                  Mean              StdDev                Median                IQR            Outliers         OPS            Rounds  Iterations
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
test_pick_only            428.6290 (1.0)      8,381.7100 (3.94)       465.9007 (1.0)      245.5388 (2.86)       448.8950 (1.0)      17.9607 (1.0)         9;107  2,146.3802 (1.0)        1081           1
test_pick_and_slice     1,167.1580 (2.72)     2,127.0860 (1.0)      1,223.6495 (2.63)      85.7735 (1.0)      1,196.0865 (2.66)     59.2490 (3.30)        78;59    817.2275 (0.38)        738           1
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

This is almost 1,000 times faster if a slice is made (depending on the size, etc).