Laser engraving very slow

I try to make engraving of the photo on wood using laser that control it's power by spindle PWM.
I add g-code file for example and my configs also.

When processing going on, machine not reach maximum speed. Machine decreese speed at almost each point and it's movement very sharp. What cause such deselerations? Why laser can't be moved smothly.
It seems that G64 doesn't influence on work.

I've made some test and get such results:
- slowness of the movement assousiated with speed of spindle S### (look my G-code). And if I even write in each line the same speed, for example,
X### S45
X### S45
X### S45, then it will also make some delays in that points, not looking at G64 (Maximum speed). If I delete from G-code S### codes, machine works as rocket (exaggerated asertion).

Now is released new 2.7.15 version of linuxcnc, I upgraded to it and got the same results.

I suppose this problem is better to solve in program code of linuxcnc, Spindle speed with PWM should also have some dependencies to G64, spline change and [TRAJ] as coordinated.

Maybe somebody have other thought for solving this problem:
- maybe it's possible to set PWM power of laser, using some new Axes, but then can't to regulate it separately from speed.
- or maybe I have some wrong settings in the HAL (wrong use of servo-thread and base-thread)

Maybe it's not cleer what I want to get.
I want to engrave with laser some photo, I at video in example link (from time 1:00):

Problem was solved by using M67 (Analog input synchnize input).
like this: :woohoo:

Changes in S seem to break the lookahead. I am not sure why, but as you have noticed, using M67 is better.

One little tip for the k40 Laser engrave DRW editor if you plan on designing anything in it, you can use "solid white" objects placed where you want to "erase" other lines or parts of other objects. Since it's my only vector editor I have taken this to a ridiculous degree. Very tedious but works.