OK, I think the first thing to say is that you aren't at fault here.
Folk work at different speeds and it sounds like your school has adopted a rather awful way of teaching and assessing students. It's ableist and unproductive.
I want you to know that.
But, here are some things that might help you keep up when you need to.
The first trick is ti always start with a quick plan. Whatever the assignment, give yourself a few minutes at the start to plan the task and try to break it down into chunks.
For example, if it is writing an essay, you would break it down into an introduction, a main body and a conclusion. Bullet point list the main points of the essay you want to hit too.
If you are comfortable doing do, take how much time you have and divide up across the plan. It might be something like five minutes for the introduction, three minutes for each point if the essay and five minutes for the conclusion... And practice sticking to that.
As a teacher, I'd always look at student plans, especially if they had run out of time, to see if they understood more than they could answer in the time they had. Most did.
Another thing I'd reccomend is perhaps talking to your teachers. Find one that you feel comfortable with and ask them for advice. Generally, teachers want to support you in learning. If you di well it looks good on them.
I do hope things get better for you. I know it might knock your confidence, but it really is no reflection of your ability.