Why bgsave on redis make data read slower?

Bgsave command is a command to fork redis a new process and do background saving on a new core of processor. However, my 2GB data on redis (about 1 GB for RDB after save) is slowdown my data transfer from redis to my app. I tested with 4 core machine (2.5 GHz CPU) with no significant CPU usage from other process. After I run bgsave, I run hgetall immediately and it delay about 14-18 seconds before data is showed in redis-cli. Is there a way to handle this problem? Do I need to use replication to another machine just to do a bgsave command and leave main redis with no bgsave?