There exists a large table in SQL Server with almost 40.000.000 records. it required lots of time to running queries onto it. therefore we made the decision to depart all old and outdated records there and move functional records inside a new table an make use of the brand new one. although it said to be faster than old table that have almost double count of records, the problematic result was somthing else. although runnig a question on old table takes nigh 45 seconds,around the new table with same structure (secrets and indexes) and half count of records, it requires over 30 minuts to running same query. where we've mistake or skipped some important issues?

Have you make sure to migrate the indexes in the bigger table and re-generate statistics around the new table?

Otherwise, start there. That's usually problem in these instances (particularly with large tables).