Trying to fix Trackster on our Galaxy, I noticed that my browser (Firefox) catches a lot of warnings, also on properties of CSS that I thought would be especially for Firefox.
Can this cause lagging while using Galaxy? (some of our users experience slow browsing on our Galaxy).

No, these definitions will not cause lagging; they are for older versions of Firefox. 

Using more Web server threads may help reduce the lag that users see:

http://wiki.galaxyproject.org/Admin/Config/Performance/Web%20Application%20Scaling#Web_Server.28s.29

Best,
J.