Huge loading time of my website

Hello,
I have a website hosted by a service provider in Germany (https://kam-webagentur.fr), except that I noticed a very slow loading of web pages, I contacted him but he told me that 'this is server cache.
Could you help me please ?

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First and foremost, server cache is not responsible for the huge loading time of your website. In fact, it is quite the opposite. Let first understand what server cache is exactly.

Basically, there are four different types of web caching i.e. site, browser, server, and micro cache.

Server caching is one of the finest methods for reducing server loads. Suppose a request is made, the server immediately examines its temporary storage for the required content before processing the request completely.

If the content that is requested is present in the server cache, it will be sent to the browser immediately. This facilitates your server to manage more traffic and return your webpages quicker.

Hope this helps.

The web hosting provider told me that's redis cache server
Could someone help me to find a way how optimize the server ?

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Find a provider who has servers locally is the best thing you can do initially.

Minimize HTTP requests.
Minify and combine files.
Use asynchronous loading for CSS and JavaScript files.
Defer JavaScript loading.
Minimize time to first byte.
Reduce server response time.
Choose the right hosting option for your needs.
Run a compression audit.

Cache should speed up your website not slowing them down (((: very strange support I thing who told somethign like that, support mostly proposing enable the cache on the website ...
Anyway, I dont know what website do you have is it a CMS like wordpress or different solution, if Wordpress the first steps you should take is to find a plugin called AutoOptimize, its doing a lot of things to speed up your website, lets say if your website loads 6 seconds it can reduce that time even to 3 seconds without doing anything except installing of those plugin and configure them. Of course there are many more ways of optimize your website to speed them up but it needs some coding someties so it may not be cheap way.

Run a compression audit The smaller your files, the faster they'll load — and the lower your overall load times will be. Pages with lots of images and other content can often end up being over 100KB in size. As a result, they're bulky and slow to download. You can speed up download times by compressing them.

Look into minifying any JS and CSS code

Ideal Website Load Time – 2 to 5 seconds. However, each second beyond 2 seconds results in greater bounce rates. In fact, 40% of polled internet users report abandoning a site if it takes longer than 3 seconds to load. Moreover, 47% of users expect desktop sites to load in 2 seconds or less.

Hey,
I would like to advice you that you have to change you web hosting provider. Because I think so your currently provider has no strong server to control the load of websites. Another advice for you
You have to make sure that your website have low weight images because heavy weight images decrease the website speed.

Large image files or a lot of images can take up a lot of your page load time. Make sure all your images are resized and compressed correctly.

Huge loading time affect website ranking & search engine rankings.

Try to optimize your media files, js files, css files , etc. Otherwise your website ranking may also get hurt.