On Mac OS X,I get

tim$ python
Python 2.5.5 (r255:77872, Mar 21 2010, 22:08:39)
[GCC 4.0.1 (Apple Computer, Inc. build 5367)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import bsddb
>>> print bsddb.__version__, bsddb.db.version()
4.4.5.3 (4, 6, 21)
>>>
tim$ /opt/local/bin/python2.6
Python 2.6.5 (r265:79063, Apr  8 2010, 22:42:38)
[GCC 4.0.1 (Apple Computer, Inc. build 5370)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import bsddb
>>> print bsddb.__version__, bsddb.db.version()
4.7.3 (4, 7, 25)

So the database versions are:
python 2.5 bsddb 4.4.5.3 (4, 6, 21)
python 2.6 bsddb 4.7.3 (4, 7, 25)

On python 2.5:
Python 2.5.5 (r255:77872, Mar 21 2010, 22:08:39) 
[GCC 4.0.1 (Apple Computer, Inc. build 5367)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import bsddb
>>> dbenv=bsddb.db.DBEnv()
>>> dbenv.open(".", bsddb.db.DB_INIT_TXN | bsddb.db.DB_INIT_MPOOL | bsddb.db.DB_INIT_LOG | bsddb.db.DB_CREATE)
>>> db1=bsddb.db.DB(dbenv)
>>> db1.open("note.db",flags=bsddb.db.DB_RDONLY,dbtype=bsddb.db.DB_UNKNOWN)
>>> 

and on python 2.6:
Python 2.6.5 (r265:79063, Apr  8 2010, 22:42:38) 
[GCC 4.0.1 (Apple Computer, Inc. build 5370)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import bsddb
>>> dbenv=bsddb.db.DBEnv()
>>> dbenv.open(".", bsddb.db.DB_INIT_TXN | bsddb.db.DB_INIT_MPOOL | bsddb.db.DB_INIT_LOG | bsddb.db.DB_CREATE)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
bsddb.db.DBError: (-30971, "DB_VERSION_MISMATCH: Database environment version mismatch -- Program version 4.7 doesn't match environment version 4.6")
>>> 

The incompatibility between the two environments is therefore resolved as being due to different versions of bsddb. Thanks for all your help in determining this.

The database slowdown still remains to be resolved.