Partitionining perfortmance degraded drastically after upgrading the database from Oracle 102.0.4 to Oracle 11.2.0.3

I have upgraded one of my 2TB databases with partitioned tables & indexes from Oracle 10.2.0.4 to Oracle 11.2.0.3 recently and experiencing severe poor query performance. The response time of the queries have increased by 20-25 times.

Before the upgrade to 11g

PGA_AGGREGATE_TARGET=6GB
SGA_TARGET=4GB
SGA_MAX_SIZE=6GB

After the upgrade to 11g
MEMORY_TARGET=6GB

PGA_AGGREGATE_TARGET=0
SGA_TARGET=0
SGA_MAX_SIZE=1GB

My understanding is that if I set SGA_TARGET and PGA_AGGREGATE_TARGET parameters to 0 then Oracle manages theses as per the MEMORY_TARGET parameter in 11g. Q. Is it true that the SGA can't grow more than 1GB since SGA_MAX_SIZE is still set to 1GB?

Q2. Will the performance improve by setting  SGA_TARGET to 4GB by default?
Q3. Should I also set the PGA_AGGREGATE_TARGET to GB as it was prior the upgrade?
Q4. Any known issue of partitioning performance impact after you upgrade a 10g database to 11g?

Please let me know if you any suggestions to resolve this issue

Appreciate your help in advance,
Ashoke

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