Nobody replied. So I wrote to the Magix support team and got a solution/workaround.

Question 1. After I rotated a video, how I can crop the video the way how I rotated?.

Answer: This seems to be a bug. I passed this to Magix our developers. 
But there is a workaround. If you just want to use a 90° degree rotation, you can use the corresponding icons for that instead of doing it manually, and then it works properly. I have attached a JPEG file that shows what I mean. 

Question 2 After I cropped a video why it fits in the screen automatically? can I keep the object in the same size as I cropped and with Same Y-Pos, and X-pos?

Answer:  You can change this behavior by unchecking the "Fullscreen" box in section mode. I have attached a jpeg file that shows what I mean. the checkbox may not respond properly but it will be ok when you begin to crop.

Question 3. Why there is a lag when I change the size? it's happening randomly. my system is 24gb ram, AMD Radeon HD 5450, 17- 7700

Answer: I feel like the support agent somehow tried to escape from this question. given below is his 1st replay. then I asked him how he wishes to proceed with this issue. but he ignored this question in the next email.

" That's not easy to say, unfortunately. There can be several reasons for this and it can also occur with powerful systems. For example, the system can be slowed down by background programs. Of course, it can also simply jerk from the program. I would have to take a closer look at your computer to be able to make a binding statement here, but I would suggest that we check off the first two points first."

What I learned. Magix customer support is very co-operative. I'm satisfied with their answers. My major issues are resolved. but They are very slow. they may take 3 days to respond. you may don't get support at the weekend. each subsequent email will take again 1 day to reply. So don't wait for their replay if you have to submit your work within 3 days.

@Basheer

Hi. I did see your original post but didn't understand how you explained your problem.

I think the automated reply you get on your enquiry to Magix support normally explains it can take up to 4 days to get a reply, but it varies. I have had the occasional reply on the same day but some take the full four days.

When you post questions here it helps to give as much information as possible including things like hard drive sizes and speeds. What type of file you are working on. File type and resolution.

Do you have the latest driver support for all parts of your system?

What operating system are you using?

The freezing of the program during playback can have many causes.

From where is the projects data being stored and sent to.

How large are the files you are working with.

How much free space is on the working drives.

How much ram is the project using.

How much work is the graphics card doing in 3D rendering.

Are you reading and writing to the same disk.

What additional effects may be in play.

You may think you are just resizing and moving an image around a screen but that has to be calculated in real time and at the resolution of the video at the frame rates used. If you add an effect into that clip, that will be additional calculations going on.

That is why they can't say for certain what the cause of freezing would be.

My projects can use up 32 gig of ram for a video that is only 15 secs long but it varies depending on what I'm doing within MEP.

The type of hardware also plays a part and which parts of the system that come into play depends on what is being asked of the program to perform which task at specific points in time during the export, which is why sometimes it will speed along and at other times it will slow down.

I hope this explains why the people at Magix may not have given what would seem like an adequate reply to that question.

Ray.

Thanks for your reply. I will try to reply to your questions inline below bold.

When you post questions here it helps to give as much information as possible including things like hard drive sizes and speeds. What type of file you are working on. File type and resolution.

many Questions Like below have been asked when I opened the ticket in Magix. they did not ask any further questions related to my system.

Do you have the latest driver support for all parts of your system?

Yes

What operating system are you using?

Windows 10

The freezing of the program during playback can have many causes.

From where is the projects data being stored and sent to.

C:

How large are the files you are working with.

The project file is 4MB, Video folder size is under 14GB

How much free space is on the working drives.

Its 1TB SSD, More than 200 GB free

How much ram is the project using.

I did not notice.

How much work is the graphics card doing in 3D rendering.

<20MB.

Are you reading and writing to the same disk.

Yes

What additional effects may be in play.

Mainly keyframe, Crossfade, Texts...

This issue is encountering randomly, Same project files work perfectly sometimes.

Have you ever faced the same issue?

Hi.

When you ask MEP (or any video editor that is non destructive to the original files) It will try to load the complete project into ram. If it can't do that because the operating system needs a chunk of it and MEP needs a chunk of it to work, then the rest gets put onto the hard drive again.

So effectively a copy of the whole project, but including instruction sets of what to do to the video as it renders the project as it goes along. Reading from the disk is much slower than pulling the data from ram.

If your hard drive is fragmented and the data it's reading is spread about, that can also cause a slowdown, but not as much as if you were using a spinning drive.

It is always best to use three hard drives ideally, or at least two. Projects and exports should never be sent or read from your C: drive.

Although SSDs and M.2 drives can be fast, equally they can be slowed down a lot by moving around large file sizes.

Some effects can be huge 3D rendering hogs, depriving the GPU of resources for video decoding or encoding. That process may then be switched to the CPU which is even slower and has to be done at that point by software.

I know which of my effects slow down or load the program to a point where it slows down and on occasion when there is a lot of background calculation going on, such as with moving and altering graphics, then yes the program can freeze for a few seconds or slow down exports to less than a frame a second. I can also watch the resources of my PC going crazy trying to process the commands I've given it.

Here you can see my nvidia card doing a lot of 3D rendering on export allowing the Intel to have ore resources available for video encoding. Even so,there is so much 3D rendering being done the Intel GPU is also doing a fair bit of 3D work itself.

Above is typically what is happening on a graphics intensive bit of the timeline between the two GPUs. When one GPU spikes the other has to slow down. Both can't run at once on most machines as motherboard architectures are not normally configured that way.

The more 3D work the Intel GPU has to do, the less time it has to apply to video Decoding and the slower the export becomes.

You can see wherever there is a peak of performance in one GPU that there is a corresponding dip in the performance of the other. The CPU is doing very little work as the GPUs are doing quite well at what they need to do.

So performance is relative to workload and machine specs and file types being used and encoded to.

All of my screen shots come from my 4K 50fps projects and yes sometimes it takes an age to render. Recently I've done a whole batch of 13 second clips that are mainly making heavy use of graphics and have taken over 20 minutes to render each one. My machine specs are in my signature beneath.

Ray.