As mentioned in the release note of 0.9.4, it seems the size of the vault may cause some performance problems.
So I just wonder that to what extent, a vault could be considered as a large vault?
Or from the perspective of the end user, is there any suggestion can be given on how many files may I put into 1 vault?
This is a good question, and I’m not sure there is a direct answer. This is partially going to depend on your computer specs and what features you use in Obsidian. For example, the graph has been tested on very large vaults of 30,000 notes.
And Obsidian is getting constant improvements. For example, the graph now renders gradually, and there are improvements coming to link auto-complete.
I even tested vaults of 80K+ notes and this is working great on my computer too.
I am using a research vault of 85K+ press articles, combined with 35K+ blog articles and another 20K+ notes. In total 140K files in 3 subdirectories of one vault. I can certify that this works too.
The graphs take some time. Think this is totally understandable.
Some issues with links and backlinks who seem to have some issues (seems is the word here. Investigations stille needed).
As Sam rightly states this would not be possible on a Raspbery Pi 
Thank you for providing the example.
Comparing to your vault, I think there is no need for me to worry about the size of mine at all.
I do believe the Devs do a great job and they definitely care about performance.
Another example: I made a Natural Language Analysis (Python) in the 85K vault which creates a big Markdown file (2,1MB).
Obsidian reads that file without a problem. Typora DOESN’T -> shows me a white page. Joplin takes much longer to read the page. Sublime Editor is as quick as Obsidian but but after all they are known for this.
Obsidian is not only great at handling big amounts of files, it also handles big files at a very (very) good speed.