can decode / transcode exist at the same time ?

1.  I created

MFXVideoSession *g_mfx_session_d3d9;
MFXFrameAllocator *g_pMFXAllocator_d3d9;

use d3d_allocator.cpp  for transcode 

and not release the session,
because if recreate (like restart ) by release session 20 to 30 times the IQS will crash ? WHY?

2. then I create  

MFXVideoSession *g_mfx_session;
MFXFrameAllocator *g_pMFXAllocator;

for not transcode mode use sysmem_allocator.cpp

and then weird thing happened,  If I start transcode first , I get best speed about 170fps 1080p

but then I use system memory mode the speed low to some about 10fps(but only run system memory mode it will be 70fps 1080p)

So, Can someone explain the phenomenon, what should I do?

Thanks