The design looks pretty good. I agree about changing the font size: 12px is much too small on modern computer screens. Try bumping up the font-size to at least 16px, and set the line-height to 1.5: it’ll drastically improve the readability of the site.
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Regarding the slider:
(1) It’s highly unlikely that anybody’s going to open your page and sit there for 30 seconds watching the slider change. You might want to consider cutting it back to three or four slides. That way, you still get some movement and visual interest without loading images that few are going to look at.
(2) The mole, rat and rabbit control images are incorrectly linked: they’re missing the leading https://, which means they go nowhere.
(3) The flea and cluster fly images lead to non-existent pages.
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I’d really make an effort to speed up the site. One of the problems is that it takes a fair while for a visitor to see anything at all. Based on a Webpagetest.org analysis, it takes about eight seconds before your website begins to render for a new visitor, which is a long time to be looking at a blank screen.
A few basic tips that may help you in improving the performance of the site:
(1) Reduce the slider to just a few images, as I’ve described above.
(2) Install a caching plugin such as W3 Total Cache.
(3) Use EWWW Image Optimizer to losslessly reduce the size of your images.
(4) A slightly more advanced step would be to use CloudFlare, which often helps to improve site performance.
