It was a fairly explicit design goal. By setting up Congress so that one house mirrored population and the other mirrored number of states, they basically guaranteed that there would be no substantial legislation passed on the issue of slavery, at least not until a major shift in culture.
In a broader sense, remember that "fast and agile" is another way of saying "changes rapidly". Since government writes the rules you have to operate by, both in your personal and economic life, it's important that those rules not change quickly if the changes are at all controversial.
You do not want an agile and fast incompetent government, that's for sure.
I'm perfectly okay with Congress deliberating on a piece of legislation for an entire year. Some issues are hard.
attributed to Thomas Jefferson.
Having one party control everything leads to shenanigans. Trust me, you do not want an agile and fast government.