DAX formula optimization question

The transaction table has ~500,000 rows - and will progressively get larger. At the moment, I have two formulas set up - one that identifies a range of days between requested and actual ship dates, and assigns a text statement (e.g., <15 days, ontime etc.). The next one counts the number of rows with the statement. 

1) Late Delivery 30 days = IF('Sales Transaction'[Days Requested v. Ship Date] >=30 && 'Sales Transaction'[Days Requested v. Ship Date] <=44, "Days Late (30+)", "N/A")

2) Count Late 30+ = CALCULATE(COUNTROWS('Sales Transaction'), FILTER(ALL('Sales Transaction'), 'Sales Transaction'[Late Delivery 30 days] = "Days Late (30+)"))

After that other formulas calcualte off of these, such as% percentage of late/ontime shipments by category bins (15 days, 30 days etc.).

Originally, I had the first formula as a long IF statement for each bin, but it was too slow, so I split the IF statements.

This might not be the best way to do this - so if you have any suggestions, please let me know. Honestly, DAX is not my forte! 

In the meantime, thanks for your help so far!