Is there a tremendous decrease in suffering at stream-entry?

One who has attained the fruition of stream-entry or has arrived at such right views & convictions which generate the path for the attainment of fruition of stream entry is a great person who dwells in relative comfort.

One who goes beyond doubt lives with verified confidence and is considered a rich person even if wothout money.

In some ways this is a trick question, because the meaning of suffering in the Buddhist sense is different from the meaning of suffering in the colloquial English sense. In the Buddhist sense, suffering is a mental phenomenon: an attachment that drives us and constantly reproduces the conditions we live in. When that kind of suffering is eliminated, it doesn't necessarily do much to change the conditions of our life in the short term, even though those conditions might be what we colloquially call 'suffering'.

So for a silly example, imagine a man who is obsessed with getting through a door, so much so that he hits it, kicks it, screams at it, throws himself bodily against it, bangs his head on it, over and over and over. Before stream-entry, that man is completely unified with the obsession of getting through that door: it is who he is and what he does, and he suffers for it in both the Buddhist and colloquial sense. After stream-entry, the man disentangles from the obsession. He sees the obsession as something separate from him, he understand that the overwhelming desire to get through the door is intrinsically hollow. He may still throw himself at the door out of habit; he certainly still feels the aches and bruises from his past efforts. But the mental (Buddhist) suffering abates quickly. He may even laugh at himself, and at the absurd act of banging his head on a door that he finds himself in the middle of.

Once one realizes the hollowness of the attachment, the karmic manifestations — that recurrent urge to kick and bash and scream against obstacles in the world — start to abate, but they may persist for some time and cause further worldly problems. But one doesn't suffer mentally over them in the same way, or to anything like the same extent.

You quoted the Nakhasikha Sutta which states that suffering that remains for a person who has attained stream entry is like dirt under the fingernail, compared to suffering remaining for an ordinary worldling which is equivalent to the great earth.

For e.g. a person's immediate family members all die due to an unfortunate accident. This person may become very depressed and stay depressed for a long time, if he is an ordinary worldling. He would experience sadness (aversion) and delusion - two of the three poisons.

A stream enterer would still become depressed, but then he will quickly come to his senses and remember the teachings of the Buddha, that all is impermanent.

He is aware that his sorrow is caused by clinging to the idea that the deceased family members belong to him (the self, an idea conjured by the mind).

He understands how the idea of the self arises, due to dependent origination. He understands how dependent origination works.

He also understands that the notion of persons and family members are all reification/ objectification-classification/ concept proliferation (papanca), just like the idea of the self.

The stream enterer would be able to let go a lot sooner than the ordinary worldling.

The ordinary worldling may get stuck in mental loops that amplify sadness and delusion. The stream enterer experiences sadness but has the wisdom to not get stuck in mental loops.

In comparison, an arahant will not suffer at all, because he has completely uprooted ignorance through wisdom. For the stream enterer, ignorance is still there, but it has been reduced greatly by wisdom.

The stream enterer still has craving and clinging, but now he has attained sufficient wisdom to understand them and how they work. In comparison, an arahant has completely destroyed craving and clinging.

Reduction of suffering for a stream enterer doesn't mean that negative events or painful sensations do not happen anymore. It simply means that the stream enterer does not suffer from them very much, compared to the ordinary worldling. His ability to endure them has increased tremendously.