Prevayler and Joafip have opposite concept.

first difference:

Prevayler is for all in memory database with backup on file ( Prevalence concept )
Joafip is to store all objects in file. Only needed object are reloaded in memory ( lazy load of object graph), then discard ( may be update in file ) when no more needed. The data size can exceed the available memory.

second difference:

it seems Prevayler use a request systems
joafip is based on "Object getObject(String key)" and "void > setObject(String key,Object object)"

third difference:

Prevayler expect "Serializable" object
Joafip have "persistable" rules ( http://joafip.sourceforge.net/presentation/ppojo.html#prules), object may implement or not Serializable/Externalizable.

The common point is persistence in file

joafip and Prevayler are object persistence framework

About speed

It is slowest access file than access object kept stored in memory:

each object access cause setting object state reading object associated record in file
each save cause object tree in memory visit to discover object added/removed/modified and then pass writing in file

About CPU usage, there is a background garbage file record collector running ( frequency can be set ), this is needed when there is a lot of object deletion and creation to avoid file grow too much.

Below link to help understanding

joafip : http://joafip.sourceforge.net/howitworks/howitworks.html
Prevayler : http://www.prevayler.org/wiki/Links