Looses some materials and if a lot of trees flowers gets very slow

In version M3, if you copy similar group of items (some minor changes made) to same file, even all contains similar materials, the new one looses material. Material is made by me. in older version, I have not noticed that.

When my file is more than 55 000 kB and contains a lot of trees and flowers, even I have managed to create small flowers and trees (see Timoko), it takes a lot of time to open or to move around, That is the case if you have saved or opened file as whole 70x70 m. If you have saved file as zoomed 10x10 m, file opens faster and you are able to move around quite fast. Worst is if hidden edges are on. Even saving takes many minutes.

And you did not observer this in M2, M1 or M0?

Any chance you have extensions installed? (Perhaps render engines?)
Have you tried with the extension disabled? Does that make a difference?