To my knowledge there is no “file conversion” tool with Movie Edit Pro 15 Plus. However, you could just load the video and then immediately export it as an MPG. File > Export movie > Video as MPEG.

But as John mentioned, that’s not really the greatest way to convert files, especially if you plan on doing further editing to the video. Is the video lagging because you’ve added various effects? When I’m using multiple effects within a video I will begin working with the settings at 16:9 (720 x 480; 29.97fps) to ensure it runs smoothly; especially if I want something to be timed precisely with the soundtrack.

To change the movie settings push “E” or else go File > Settings > Movie. The dialogue box is called “Project/Film settings” and you want the “Movie settings” tab.
I actually tag my initial file as “low res” at the very start when I name it, and then once I have everything edited, I literally do a “save as” and delete the “low res” tag. In that subsequent file I then increase the video settings.

Because I like to keep the chain of events that I’ve rendered, I also set up two folders: one for “Audio mix” and one for "Audio/Video mixdown" - and I do a “save as” into those folders for each mixdown process. You can “undo” a mixdown but I like to save the files in their respective order of mixing. It also helps keep track of the associated files that the program creates. Otherwise you end up with a miasma of files in one folder.

I don't find the above process hinders the quality, and I export my final file as wmv.

Something I find handy is found in that same aforementioned dialogue box under the tab “Project settings.” There is a space that allows you to add little notes to remind yourself what changes you have made to that given file. Although keep in mind that if you perform a “save as” it will repeat those notes. But it’s not a bad idea to keep track of things in that little box.

That’s about all I can add at this point. John already mentioned RAM.

I’m waiting for the delivery of a new computer from Dell, but in the meantime I’m still only using an old HP single core with 2GB RAM and I don’t seem to have the degree of problems you are suffering with the lagging issue. Something doesn’t sound right. Regretfully, you might consider re-installing the program. Does everything else seem to be working fine?

On the other hand, if you can cope with the lagging and edit the video the way you want it… then once it’s mixed down it should play better; and furthermore, once it’s exported it should certainly play properly in Media Player (or whatever video player you use) and it should also play properly if burned to DVD.
Although frustrations in rendering can be a real nuisance, it's the final product that paves the way.

John may be able to help you more as he seems to know his way around the technical side pretty good: RAM and Video cards and so forth.
Having more RAM with dedicted Video memory certainly helps.
I'm only using a crappy onboard Video card right now, but as I mentioned, I'm not burdened with the same dilemma you are unfortunately experiencing.