Even at night, it will take you only slightly longer to get from A to B, provided you are near the city centre.

slightly 是副词，由形容词 slight（轻微的、微小的）加上副词后缀 -ly 构成，表示"稍微、略微"的意思。在这里修饰比较级 longer，表示时间增加的程度很小。