Along with the use of Java comes a process called garbage collection. Once an Android user closes an app, the garbage collection process recycles the memory. The problem is that the garbage collectors require four to eight times the memory that it is using in order for the job to be done quickly. If the required amount of memory is not available, things slow down. Because iOS does not use these garbage collectors, Apple can get away with putting 1GB of RAM in the iPhone, and match or surpass the performance of Android phones.
So this should clear up one of those mysteries that have you scratching your head every now and then. And just in case you are curious, the answer was posted on Quora by a Glyn Williams, who received 2700 upvotes for his response.
source: Quora via RedmondPie