Some of the columns in one of my tables contain mainly nulls, others
contain very few.

I am trying to improve performance of a procedure which updates many
rows. The rows have a trigger which compares new/old values and I have
noticed that the execution speed is much slower when comparing the
columns with many nulls. eg

Compare new.ColumnWithFewNulls with old.ColumnWithFewNulls = FAST

Compare new.ColumnWithManyNulls with old.ColumnWithManyNulls = SLOW

Is this a known characteristic of nulls?
If so, is there any way to accelerate this, other than removing the nulls?

Many thanks
Mitch