Currently, the normal spawn rate is 2x of the old respawn rate, according to this announcement.
When stacking multipliers, you have to add them up, giving the total respawn multiplier. Then, in order to get what would be the equivalent to the old respawn rate, you multiply it by 2.
|Combination||Respawn Rate||Old Respawn Rate|
|Rapid Respawn Weekend (RRW)||x5||x10|
|Boosted Creature (BC)||x2||x4|
|Improved Respawn Area (IRA)||x2||x4|
|RRW + IRA||x7||x14|
|RRW + BC||x7||x14|
|BC + IRA||x4||x8|
|RRW + IRA + BBC||x9||x18|