Faster performance when using where cust_id = '123' vs cust_id = 123. Does 7.4 fix this??

We have got used to the problem that queries of the format:

select *
from customer
where cust_id = '123' are much much faster than

select *
from customer
where cust_id = 123

(where cust_id is defined as bigint).

a. Why is this.

b. Will moving to v7.4 change this so we can avoid the whole '123' casting
thing.

Tx folks,
D