The Gemara tells us that Marror, the bitter herb, mimics the enslavement in Egypt. It started out soft but became harsh. Ritva on the Haggadah explains that the work became gradually more oppressive. 

Similarly, the Gemara (Sota 11a) describes the sly way in which the Egyptians convinced the Jews to enter into service:

They brought a brick mold and they hung it on the neck of Pharaoh to create the appearance that he was also participating in the labor. And with regard to each and every Jew who said to the Egyptians: I am a delicate person [istenis] and I cannot participate in the labor, they said to him: Are you at all more of a delicate person than Pharaoh, and he is participating.

What is the lesson in the idea that the slavery started slowly and then became hard? I believe a number of ideas can be learned:

1. It is sometimes hard to react to a gradually oppressive situation. Consider how many Jews continued to stay in Germany, rationalizing it would get better soon. The hard questions Jews must ask themselves is will we be brave enough to know when to leave our host country if such a situation arises again?
2. A person can be in an abusive relationship or work situation but because it grew slowly, became used to it. The difficulty is to know when to try to work on improving matters and when to run and not look back.