For Increasing total performance of website i want to do Image compress & CSS compress. but i face issue in image compressing. image quality is not good after compressing. is it a proper way to increase performance for website?
Certainly compression in some cases is beneficial because it reduces the total load that must be transferred. However for small images that get degraded badly by compression, it is not necessary. It really comes down to the size of images, if you are using a lot of graphics then you probably have a bloated slow page and the users would probably prefer to see crappy looking graphic rather than waiting for a slow page to load. Better yet They would probably rather see relevant content instead of a bunch of pictures. As for CSS, the is very little value in compression, unless it is a very large stylesheet; in which case you probably have a bloated overstyled mess that needs more that a tiny improvement in performance to provide a good user experience.
@COBOLdinosaur has it right that it really depends on what you are doing with your images; image optimization is indeed important -- but it really hinges on what you have for images, what you are doing with them, and how bad the rest of the page is. Meanwhile as he said, if you have enough CSS to 'need' compression, you've probably done something wrong like using halfwit garbage frameworks; see bootcrap and it's kine which by themselves are three times the size the upper limit I'd allow for the CSS of an entire site! Same goes for scripttardery like jQuery... You put jQuery and Bootcrap in a project, and combined uncompressed they are three times the limit I'd allow for a site template's HTML+CSS+SCRIPTS+IMAGES not counting content. Pretty much without seeing what you are working on, it's hard to say if it's worth the time or not for you... though the links in your signature indicate outdated outmoded coding practices and endless code bloat, in which case fretting about compression is just sweeping developer ineptitude under the rug instead of addressing the deeper rooted issues first.... ... take that "Let's Digital" site, with the endless pointless CSS inlined in the markup, endless pointless JS inlined in the markup, little if anything resembling semantics, endless pointless DIV and classes for nothing, non-breaking spaces doing CSS' job, and a massively ridiculous 3 megabytes in 98 files to deliver 8k of plaintext and less than a half dozen content images. Not counting the social media bloatware, it's easily 30 times or more larger than need be in filesizes and eight times as many separate files as needed for such a simple site. Dicking around with image optimization and white-space stripping your CSS is not the answer. Basically chewing gum and bailing wire is NOT a good long term fix.
Compress the images for sure. Not sure about jpeg compressors, but this site is awesome for PNG's: https://tinypng.com I save my images full quality then run it through there and it generally cuts them down by over 50%. I can't notice a quality difference, but apparently you lose some color depth. I'm for CSS compression, as long as you automate the process. If your going to run your CSS through a compressor manually every time you make a change don't bother. Even if your CSS is tight and in check why not shave a few K's if it takes no work right?
Always go for quality over compression unless you have far too many big images, then just change those images.