I wrote code in python that works slow. Because I am new to python, I am not sure that I am doing everything right. My question is what I can do optimally? 
About the problem: I have 25 \*.json files, each is about 80 MB. Each file just contain json strings. I need make some histogram based on data.

In this part I want create list of all dictionaries ( one dictionary represent json object):

```
d = [] # filename is list of name of files
for x in filename:
d.extend(map(json.loads, open(x)))

```

then I want to create list `u` :

```
u = []
for x in d:
  s = x['key_1'] # s is sting which I use to get useful value
  t1 = 60*int(s[11:13]) + int(s[14:16])# t1 is useful value
  u.append(t1)

```

Now I am creating histogram:

```
plt.hist(u, bins = (max(u) - min(u)))
plt.show()

```

Any thought and suggestions are appreciated.
Thank you!