Delete statement plays a major role in many of the database maintenance activities. Extreme care should be taken before executing the DELETE statements
Before executing the DELETE statement it is better to check the “Estimated execution plan”, so that we can create proper indexes to speed up the execution .I have given some of the possible reasons for slower delete.
1. Locking / Blocking - If it is a production database process is having Lock / Block on the table
2. Fragmentation - The Index pages are fragmented due to excessive delete on the table (Defragment the Indexes and try again)
3. The table you are trying to delete is referred by many tables as foreign key and those columns are not indexed.
4. There might be hanging transactions on the table - Try to truncate the Log and do
5. You can also change the Recovery Mode to simple and try - but not advised in case of production DB
Also it is recommended to execute the DELETE statements in smaller batches so that the Log space & Roll back of the records can be easier as shown below
// assume you wanted to delete 100000 records, we can split them into 10 batches as
DECLARE @V_Count INT =0
WHILE @V_Count < 100000
WHERE COLUMN = Condition
SET @V_Count =@V_Count+10000