Why my iteration speed is so low?

Hi, i try to do some simulation about particle packing behavior in a rotating drum. However the iteration speed is so low as about 30 iterations per second. Nevertheless, at the beginning when the particles are far from each, the iteration speed is 500 iter/s, no matter with -j2 or without prescribed -j parameter.

## Below is my code,

from yade import pack,geom,utils
import numpy as np
from yade import qt
### the unit in the simulation is UI, like kg m s Pa
### the particle has diameter below 100 microns

muS = 0.57
muF = 0.9
FricAngleS = math.atan(muS)
FricAngleF = math.atan(muF)

density = 8150
yongmodu = 195e5 ## scaled by 1e-4, normal value is 195Gpa
poisn = 0.3
Dsph = 80e-6 ## particle max diameter
angvelsw = -25 ## rotation speed rad/s
nrot = 6 ## rotation times

matSph = CohFrictMat(density=density, young=yongmodu, poisson=poisn, frictionAngle=FricAngleS, momentRotationLaw=True)
SMat = O.materials.append(matSph)
matFacet = CohFrictMat(density=density, young=yongmodu, poisson=poisn, frictionAngle=FricAngleF, momentRotationLaw=True)
FMat = O.materials.append(matFacet)

### geometry ###
radiusSweeper = 10*Dsph
radiusSweeper0 = 9*Dsph
lengthSweeper = 12*Dsph
numSweeperParts = int(round(2*np.pi*radiusSweeper/5.0e-6))

## spheres ##
O.bodies.clear()
sp = pack.SpherePack()
x1, y1, z1 = -radiusSweeper, 0, 0
x2, y2, z2 = radiusSweeper, lengthSweeper, 2*radiusSweeper
sp.makeCloud((x1, y1, z1), (x2, y2, z2), psdSizes=[0.010e-3,0.024e-3,0.035e-3,0.051e-3,0.085e-3], psdCumm=[0,0.1,0.5,0.9,1.0])
sp.toSimulation(material=SMat)

## drum ##
Sweeper=[]
for i in np.linspace(0, 2*pi, num=numSweeperParts, endpoint=True):
 Sweeper.append(Vector3(radiusSweeper*cos(-i), 0.0, radiusSweeper*sin(-i)+radiusSweeper))

SweeperP=[Sweeper, [p+Vector3(0.0,lengthSweeper,0.0) for p in Sweeper]]
SweeperPoly = pack.sweptPolylines2gtsSurface(SweeperP, threshold=1e-7)
sweeperid = O.bodies.append(pack.gtsSurface2Facets(SweeperPoly, wire=False, material=FMat))
boxid = O.bodies.append(geom.facetBox((0,0.5*lengthSweeper,radiusSweeper),(radiusSweeper,0.5*lengthSweeper,radiusSweeper), \
                              wallMask=63, material=FMat, wire=True))

## remove the spheres out of the drum ##
for eb in O.bodies:
    if isinstance(eb.shape, Sphere):
        xi,yi,zi = eb.state.pos
        lengi = np.sqrt(xi**2 + (zi-radiusSweeper)**2)
        if lengi > 0.96*radiusSweeper:
            O.bodies.erase(eb.id)
## total sphere is 6691, facet is 2020

### define engines functions ###
t1 = 0.1
t2 = t1+nrot*(2*np.pi/abs(angvelsw))
t3 = t2+0.1
def change_motion():
    if O.time > t1:
        rotaEngineSw.dead = False
        rotaEngineSw.angularVelocity = angvelsw
    if O.time > t2:
        rotaEngineSw.angularVelocity = 0.0

O.engines=[
 ForceResetter(),
 InsertionSortCollider([Bo1_Sphere_Aabb(),Bo1_Facet_Aabb()]),
 InteractionLoop(
         [Ig2_Sphere_Sphere_ScGeom6D(),Ig2_Facet_Sphere_ScGeom6D()],
         [Ip2_CohFrictMat_CohFrictMat_CohFrictPhys()],
          [Law2_ScGeom6D_CohFrictPhys_CohesionMoment()]
    ),
    PyRunner(iterPeriod=int(1.0e6),command='change_motion()'),
    RotationEngine(dead=True, label='rotaEngineSw', rotateAroundZero=True, zeroPoint=(0,0,radiusSweeper), angularVelocity=angvelsw, rotationAxis=[0,1,0], ids=sweeperid),
    NewtonIntegrator(damping=0.75, exactAsphericalRot=True, gravity=(0,0,-9.81)),
]

O.dt = 0.85*utils.PWaveTimeStep()
O.run()

## the hardware ##
Ubuntu 18.04 on VMware 14.0 with 8 processors 32 GB memory
Main system: Win10-64
Processor: AMD Ryzen Threadripper 1950X 16-Core Porcessor, 3.40 GHz
## Yade version
2018.02b

## some yade --performance test
## 4k 20k 40k spherical particles with yade -j 1 *.py
https://drive.google.com/open?id=1nqXm75HpqhG-n0XPU7MF1G0AiBfuTcSj
https://drive.google.com/open?id=1_2TWGVauOQvFznHqdiV5R35yhqzitHvo
https://drive.google.com/open?id=1rPvseozgYY9qEW41dQXKm3iz4sORTGtY
## 4k 20k 40k spherical particles with yade -j 2 *.py
https://drive.google.com/open?id=1_z7C7jkjYy_wzXy9iNaq_N4SaI_wghSV
https://drive.google.com/open?id=1hrOEshAC4YQAEmSpCrk_IHmIwvcvjz7z
https://drive.google.com/open?id=1pcR6WAmLWeCWo0-yiRGR-2g0BzJ7rMCl

Very appreciated.
Xuesong