我看不出read_fwf在Pandas中有什么用。为什么我要用它来代替支持自定义分隔符的read_csv呢？我试着对一个大型固定列宽文件的速度进行测试，read_csv要快得多：

data = ("colum1    column2222   column3333   column4\n"
        "id8141    360.242940   149.910199   11950.7\n"
        "id1594    444.953632   166.985655   11788.4\n"
        )

colspecs = [(0, 6), (8, 20), (21, 33), (34, 43)]
data = data * 10000000

with open("big_file.txt", "w") as f:
    f.write(data)

start_time = time.time()
df = pd.read_csv("big_file.txt", header=None, dtype={"colum1": str, "column2222": float, "column3333": float, "column4":float}, sep="\s+")
print(f"--- {time.time() - start_time} seconds ---")
--- 4.0295188426971436 seconds ---
start_time = time.time()
df = pd.read_fwf("big_file.txt", header=None, colspecs=colspecs, 
                 dtype={"colum1": str, "column2222": float, "column3333": float, "column4":float})
print(f"--- {time.time() - start_time} seconds ---")
--- 77.41955280303955 seconds ---