SQL Server 2012 slower than 2008
Let me know what to check/verify


You have a performance problem. Follow a performance troubleshooting methodology like Waits and Queues to identify the bottleneck. The linked methodology shows you what to measure and how. Post here the findings and we can help with specific advice based on your actual measurements. As it is is too open and is anybody's guess. Narrowing it down to a specific issue will eliminate guesswork. 

After update

The plans are quite different. The old plan had a stream aggregate low on the stack which actually has a bad  cardinality estimate (141k vs. 108k) and the hash math further mispredicts, the other way (35k vs. 108k). The new plan does not have the stream aggregate and has accurate estimates all the way to the top. Of course, this does not explain why the old plan was executing faster. 

The bottom scans have a slightly different row number (not significant) but quite different costs: old is 2.49884 (IO 2.28979 CPU 0.20905) vs new 1.59109 (IO 1.53868 CPU 0.0524084). Again would point toward a better 2012 execution (the index rebuild has perhaps reduced fragmentation?).

What is very different is the number of threads: 32 in new (each getting ~23k rows) vs. 8 in old (each getting ~95k rows). The table is fairly narrow. It could be that the large number of threads is actually hurting performance because of much much more frequent cache invalidations. I would try:


eliminate HyperThreading in the new server config (if any) and/or 
try the query with a DOP 8.


Noticed your comment:


  Added the execution plan with maxdop 8 Query is actually faster this way


It probably is just CPUs stepping on each other toes. With SSDs in place the IO is probably next to nothing so and the table is definitely too small to warrant 32 scanners. That exchange swap is probably invalidating L1/L2 constantly.