i.e., alter index owner.index_name rebuild parallel 4;
or alter index owner.index_name rebuild parallel 8;
The above statement might has rebuild the index faster but it has left a hidden trouble.
It has set the index parallelism for a higher degree than the table.
Usually wait event "PX DEQ CREDIT SEND BLKD" are high because of this issue when the query runs parallel.
So the best advice would be to restore the old parallel degree value of the index after having used a higher degree of parallelism for rebuilding.