The average speed of RAR password searching decreased twice. What is the reason?

Basically, the problems are consequences of one fundamental problem - to check one RAR password we're need to decrypt and unpack whole archive. As this is way too complex process for GPUs it's done on CPU. So most likely at beginning you're getting 7000 speed (for example) for 1st phase of password recovery - key derivation which is done on GPU.

Once it finished 2nd phase started -- key checking by decrypting and unpacking on CPU and while it isn't done GPU doing nothing == speed reduction. If archive is large enough (and without encrypted headers) it may become a main bottleneck here.