if the left side of the equal sign is a calculation, then the WHERE clause is "non sargeable", meaning that the optimizer can't use simple search arguments and must instead build an efficient query plan and must calculate every field in your table in this matter.

Maybe something like

-- midnight today

DECLARE @dFrom datetime = dateadd(dd, datediff(dd, 0, getdate()), 0)

-- 23:59:59 today

DECLARE @dTo datetime = DATEADD(dd, 1, @dFrom)

Then, in your WHERE clause:

[datetime] >= @dFrom AND [datetime] < @dTo

See if you get a better execution plan with this.