1. Then you need to run a trace on the SQL before and after the load, and see what the extra response time is attributable to. Maybe a change in execution plan, for example, or maybe something else.

2. When you analyze the table use DBMS_STATS and specify cascade => true
Only you can answer on the impact of analyzing during the day time, but if you have made no effort to physically order the rows you are adding then you can most likely use a much lower estimate percentage than you think. Have a look at this: http://www.jlcomp.demon.co.uk/stats_i.html

3. "What's the disadvantange if just compress the partitions without update?" What do you mean here?