If you're not using LW then it won't make it any slower. However, most java based programs do tend to slow the computer down if they are being heavily used. The way to keep it from doing this is ....
(1) Keep shares below 2,000.
(2) Keep incompletes to a small number; Tip: Purge the Downloads Queue! (click on blue link).
(3) Don't use the Resume button on all incomplete files. This can have side-fx such as affect LW's connection quality. May queue your files for much much longer. This will also stress LW & your comp. If you use Resume/Find sources then just apply it to a couple or so at one time. Much better & effective process which does not put strain on your computer is to re-search the files. You can right-click a search tab & go to advanced search & choose Find More results or search again. You can also right-click any file in your download window & choose search for ? term that is similar to the file name.
(4) Close off search tabs that you don't need any more. Set both uplds & downlds to autoclear. Close all LW windows you don't need open at that time. eg: Connections window.
(5) Disable Ultrapeer capabilities.
(6) After opening LW wait 10 to 20 mins or longer before using. Let it settle down 1st.
(7) Keep your Java up to date.
(8) Don't download too many files at one time. Concentrate on the few you have going.