Large size css file slows down Dw

Please upload your site to a remote server space you control.  Validate the code and fix reported errors. 

HTML Validation - https://validator.w3.org/

CSS Validation - https://jigsaw.w3.org/css-validator/

If you're still having problems, post the URL here so we can take a look at it.

First, to the file being large, have you run a site speed test in awhile to see how your page is performing? I would recommend doing that because slow loading times can negatively impact your SEO as well: https://web.dev

Second, is the CSS file local or remote? Remote files have been known to impact DW because it is constantly trying to keep the connection to the server. If this is the case, can you load the file locally instead of remotely?

If it slows Dreamweaver down, imagine what it will do for the network when deployed.

My largest CSS file is well under 200 KB.  I have no problems.  Why is yours so huge?  

Too much weight in scripts & supporting files does nothing positive for your site or your site visitor's experience.  I think you must replace what you have now with a much smaller stylesheet.