Just look at the "health" of this BT seed or the "replication rate" mentioned by bitspirit.
The higher your health, the more people will download this file, so the more likely you are to download this file. Moreover, because BT means that the more people download, the faster the speed will be, so basically you can judge the quality of a seed by looking at the health.
What you said: What I want to ask is how to judge whether this seed will accelerate instantly when the speed is relatively slow.
It's hard to say. It depends on the internet speed at that time and your contact with other people who downloaded this file.
Only when these two meet the requirements will the speed go up.
There is also that BT will be slow to start downloading. There are more people downloading, and the speed goes up at once.