why this kind of score is very slow?

As you enter the score, it requires more computer resources and MuseScore currently recalculates the location of everything in the entire score every time you do anything to it. This will change in version 3 when it comes out so it has already been addressed.

What you can do in the mean time when editing the score gets slow is to create a second score with the same instruments, time signature and key signature. Use this to enter a reasonable number of measures. A reasonable number of measures is so that note entry does not slow down and depend on how many instruments are in the score. Copy everything from this score to the real score and take a little break while MuseScore incorporates all of the measures into the score. Delete everything from the second score and repeat the process.

You may also want to open the Preferences and make the auto save time longer than 2 minutes since saving a large file takes time and may take 2 minutes if the file is large enough. You will not be able to do anything in this situation. I personally turn the auto save off and remember to save after every paste in when using this process.

There is one last thing to consider though much of this is suspect does not apply to traditional Chinese music. System items like rehearsal marks, tempo changes, key changes and time changes do not copy. You will have to work through the slow edit of the main file to add these to it. I suggest that you always make time signature changes before you paste to avoid having notes turning into tied notes when you fix the time signature.