Which plugin is slowing WordPress site?

If you use Firefox, there is an extension called Y-Slow which will tell you. You need to have Firebug installed first.

Aha. And what’s best (normal) grade for website with Yslow and Overall performanse score??

Try using this site:
https://ismyblogworking.com/

Try it disabling 1 plugin each run. There really is no best grade as each website is different based on content and server.

Your blog seems slow. It took 1.98 seconds to generate the page.
Your feed seems slow. It took 3.38 seconds to generate the feed.

whats the problem??

As you did not provide a link to your site, would have no idea.
How was that generated? Did you disable plugins and re-run? Did you try with the Default theme? Is caching enabled? How large is the page, images, sound files, other content? Are you on a shared or dedicated server? What Host? What version of PHP? How large is your .htaccess?
Have a few hundred others but that is a start.

This site is https://www.sagud.com. and yes i tried to disable all plugins and then again it’s the same.

First thing is the size of the images. 477K. Waaayyy too much.
The Home page is so big that a dial-up (still a large % of the Internet) would have to wait like 2 minutes to download.
Beyondthat if you answer my questions could help further.

host: mochahost
Shared server
site uses 295 MB
Php 4

The size of he page and images are the first thing that need to be fixed.
Second, Contact your host and see if you can use PHP 5. If you are running on Apache (You did not answer that one) you can usually add to the .htaccess for PHP5:
AddHandler application/x-httpd-php5 .php

Whats the average size of images on first site??..I mean what do you think is the best size of images on website?

Try this plugins:

https://plugins.sonicity.eu/compression-plugin/

https://www.poradnik-webmastera.com/projekty/db_cache_reloaded/

https://dialect.ca/code/wp-smushit/

There is no average size. Each person has their own preference between appearance, functionality & speed. Depends on your audience as well.
Your header_bg is very large by itself. Do you need it that big? And think its a slideshow maybe. and then others add to that. There are a lot of people still on slow connections. Can you afford or mind losing them? They are not going to wait a long time for it to load. Those plugins mentioned might help but may not. Some image files may already be compressed and there is not a lot of html to compress. Caching plugins help but really more towards loading time. And you still did not answer my other questions which after file sizes are the most important items.

My size of images is smaller now.. why is my site still slow?? :S

Images are just one part. I can not provide you with answers because you STILL did not provide me with answers to the questions I asked.
guess you will have to deal with it. Tired of asking ad not getting anything back.

How can i see if i’m running apache or not?