Here is a tutorial about jpeg compression by Corentin Derbré. We will se the all 9 levels of jpeg compression, their strengths and weaknesses.

First let's take a picture of landscape - 1.1Mo

Then let's save it with the max JPEG quality (9), the result is impressive in terms of wheight loss - 373Ko

Then let's try with quality 8, some small details get lost but still reliable - 238Ko

We start to see gradients errors in the sky. the edges are doing quite well - 193Ko

Quality 6 has some compromising errors in the clouds for example, but the front wave is looking fine. thus there's not much weight loss - 182Ko

 The edges become troubled but it is decent for the compression gain applied (5). Quite a step away from previous level in terms of weigh but also quality - 152Ko

We start to see some unpleasant things on the far beach and small artifacts on the edges, quality 4 has not much weigh loss - 139Ko

We have some annoying artifacts appearing everywhere, the edges are all jagged up, but it is a big weigh loss - 97Ko

Here again with quality 2 the artifacts are too much important, the back clouds look really bad for example - 73Ko

We maxed the compression, and this time there are some really disturbing artifacts not only in the back but in the front, and surprisingly the weigh is not so small as we would think with such a level, as if the max compression created some additional weigh - 69Ko