Database lookup step with enable cache for large data sets is substantially slower in 5 vs 4.4. Pentaho 5.0.1-Stable was used, as well as 5 trunk as of today. Comparing against 184.108.40.206.
Attaching two transformations, one to populate sample data, the other to test the database lookup speed.
I tested with Postgres 9.3 and SQL Server 2008 R2.
PostgreSQL ( 23x slower)
DB Lookup in 4.4 takes 55 seconds ~ 74,000 r/s
DB Lookup in 5 takes 1380 seconds ~ 2,897 r/s
SQL Server (26% slower)
DB Lookup in 4.4 takes 270 seconds ~ 14,709 r/s
DB Lookup in 5.0 takes 341 seconds ~ 11,701 r/s