Update terrain too slow

I got that terrain.update() takes too much time in one frame.

My computer:
os: windows 7
chip: Intel® Core™2 Duo @ 2.93Hz
graphics card: nvidia geforce 210

I use terrain with these values below:
heightmap 1025 x 1025
blocksize 64
near 256
far 512

When I move camera, terrain update takes about 1 second each frame.
I have read some thread in forum using some tricks to use terrain.update when need. But at frames which call terrain.update(), it s still lag…

So I think terrain.update() should run background or in many frames…

I have tried with threading module of panda3d to run in background but it’s failed…