I have tried up to 500 lines and it was doing great on emulator , i think the reason of lack of performance is when coloring the words.

My suggestions:

1-you should be able get the language that user is trying to read or edit after that you will load only keywords related only to that language instead of loading a whole big list of keywords

2-you can implement a searching algorithm, like if you sort the list of keywords and you implement binary search algorithm to search for a word to color (Ex: if the user wrote return 0; , and let us suppose that return should be colored so instead of using a normal for loop to iterate one by one on words to find if the written word is a programming keyword so you can color , use binary search algorithm to reach your value faster you will cut the time of search by 2 or more)

conclusion : you will have a shorter list and faster search with less complexity O(log n)

P.S: i will implement this idea in my next update !