Some built in column formulas like Col Mean() are very fast because they only calculate expensive values once and return or modify them for each row. With custom formulas this is harder to achieve because each row is evaluated separately. Sometimes using a separate column to store intermediate values can speed things up (for example storing the row indexes that meet a certain criteria, instead of re-calculating them every row), but that gets messy and hard to follow. If there were some concept of a column context such that the evaluation of one row could access intermediate values calculated in the previous row, then a script could calculate results for all rows at once and then quickly return them and pass the intermediate results to the next row. The formula would also need access to the whole column instead of just the current row, so the column reference should be passed unevaluated.
