python: reading messages very slow

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I'm new to using python - and have hit a giant speed bump trying to integrate a python library into ROS indigo. I created a node in c++ that processes a point cloud, and transmits segments using the following two messages:

Segmented BlobArray.msg

Header header
int16 full_image_rows
int16 full_image_cols
SegmentedBlob[] blobs

SegmentedBlob.msg

int64  id
int16[]  bbox
std_msgs/Int16MultiArray pt_list

On the python side, I have to rebuild each segment within the image:

for pp in range(0,data.blobs[Bid].pt_list.layout.dim[0].size):
    row = data.blobs[Bid].pt_list.data[3*pp+1]-ymin;
    col = data.blobs[Bid].pt_list.data[3*pp]-xmin;
    cv_image[row,col] = data.blobs[Bid].pt_list.data[3*pp+2]

The process works, but the transfer is extremely slow. The for loop itself is not the issue. Adding random numbers to cv_image for the same number of times is still quick. Only once I access the ROS message does it get slow ( 4 sec delay). The same process in C++ has a <10msec delay.

Are their python tricks for accessing these types of messages? Or maybe there are easy ways of transmitting multiple segments in an image?