Recovery model from Full to simple and Shrink transact log result on poor performance
Seen this happen a bunch.  Your first mistake was to shrink the transaction log.  When you set the recovery model to Simple the log is automatically truncated after a full backup of the Database.

What you have now is a combination of Index Fragmentation and Statistics needing to be rebuilt.  I would high recommend Ola Hallengren's free utility that can do both for you.

The following works for me after creating the procedures:

EXECUTE dbo.IndexOptimize
 @Databases = 'USER_DATABASES',
 @FragmentationLow = NULL,
 @FragmentationMedium = 'INDEX_REORGANIZE,INDEX_REBUILD_ONLINE,INDEX_REBUILD_OFFLINE',
 @FragmentationHigh = 'INDEX_REBUILD_ONLINE,INDEX_REBUILD_OFFLINE',
 @FragmentationLevel1 = 5,
 @FragmentationLevel2 = 30,
 @UpdateStatistics = 'ALL'


The second thing that happened is most likely your query cache got purged and it now has to build new statistics and has cause parameter sniffing on your queries.  Rebuilding your statistics should help.