Your database doesn't seem to be all that big. I've worked with Kentico DBs that were multi-gigabytes in size and even then the checkin/out of pages didn't take but a second or less. How many pages do you have in your content tree? Are there any CPU-intensive event handlers or scheduled tasks running? Do you have many content editors working at the same time? How does Kentico checkin/out perform on a development machine?
In addition to looking into those things, I'd check the Debug application and see if there are any issues with background threads, SQL queries, etc.
Also, you can keep your database size in check by limiting the number of page/object versions stored. Make sure you're pushing out staging tasks to your targets, or disable staging if you're not using it. These things can really blow up a database. But like I said before, your DB is not all that big.
You can even try opening the browser in incognito mode or clearing your browser cache to see if things improve. I do that from time to time when Kentico seems to get sluggish, and most of the time it works.