Making writing to a file process more efficient - python-3.6

Question

I am new to programming and i am running this script to clean a large text file (over 12000 lines) and write it to another .txt file. The problem is when a run this with a smaller file (roughly around 500 line) it executes fast, therefore my conclusion was it is taking time due to the size of the file. So if someone can guide me to make this code efficient it will be highly appreciated.
input_file = open('bNEG.txt', 'rt', encoding='utf-8')
l_p = LanguageProcessing()
sentences=[]
for lines in input_file.readlines():
tokeniz = l_p.tokeniz(lines)
cleaned_url = l_p.clean_URL(tokeniz)
remove_words = l_p.remove_non_englishwords(cleaned_url)
stopwords_removed = l_p.remove_stopwords(remove_words)
cleaned_sentence=' '.join(str(s) for s in stopwords_removed)+"\n"
output_file = open('cNEG.txt', 'w', encoding='utf-8')
sentences.append(cleaned_sentence)
output_file.writelines(sentences)
input_file.close()
output_file.close()
EDIT: Below is the corrected code as mentioned in the answer with few other alteration to suit my requirements
input_file = open('chromehistory_log.txt', 'rt', encoding='utf-8')
output_file = open('dNEG.txt', 'w', encoding='utf-8')
l_p = LanguageProcessing()
#sentences=[]
for lines in input_file.readlines():
#print(lines)
tokeniz = l_p.tokeniz(lines)
cleaned_url = l_p.clean_URL(tokeniz)
remove_words = l_p.remove_non_englishwords(cleaned_url)
stopwords_removed = l_p.remove_stopwords(remove_words)
#print(stopwords_removed)
if stopwords_removed==[]:
continue
else:
cleaned_sentence=' '.join(str(s) for s in stopwords_removed)+"\n"
#sentences.append(cleaned_sentence)
output_file.writelines(cleaned_sentence)
input_file.close()
output_file.close()

Answer 1

To have the discussion as answer:
Two problems are here:
You open / create the outputfile and write the data in the loop - for every line of the input file. Additional you are collection all data in an array (sentences).
You have two possibilities:
a) Create the file before the loop, and write in the loop just "cleaned_sentence" (and delete the collecting "sentences").
b) Collect everything in "sentences" and write "sentences" at once after the loop.
Disadvantage of a) is: this is a bit slower than b) (as long as the OS di not have to swap memory for b). But the advantage is: This is much less memory consuming and will work no matter how big the file is and how less memory in the computer is installed.