Since CSS does not change the file size of the image, then you are displaying a smaller version, but not actually providing responsive images. For SEO, no impact as long as your ALT TAG is set to something descriptive for impaired users. Using appropriate keywords in the text can boost SEO some, but only if they fit the description. Such as "front panel Cisco xxx switch", it is descriptive for users and contains keywords Cisco and switch. Don't force it though. Organic is better.
Now speed is another issue. The image sizes you list would not be excessively large unless you render them in really high definition. If you've come to a compromise between file size and quality, say a medium sized image, then loading speed is not impacted. And for thumbnails you do not need high definition. For the human eye, unless your image has fine details or is a high quality photo, users will not detect differences in quality between the majority of pixel resolutions. Only the very, very low quality would produce a noticeable pixelation. And sending a very high quality image as a thumbnail to a mobile user is a waste of bandwidth. You could consider creating actual responsive images, so that users on a variety of devices will all have a high quality experience. You can generate multiple resolutions and have your web server display the appropriate one after determining device type and connection speed. There are also image services which will do this for you and most, like Cloudinary, have a free tier.