I have made part of a mission with a tank and about 20 guys on the map. The mission lags when ever I play it. I don't have this lag issue with any other mission custom or original. Any ideas of what could be lagging the mission?

The lag is normally caused by the amount of calculations your system has to perform. Anything that strains that will tend to give you slow-down and lag. Rain, tanks, lots of men all contribute to this, as do lots of bullets flying and lots of scriptblocks running in the background.

To avoid it without removing some of the tanks and men (you should be able to get 20 men and a tank with no problem at all), try reducing the amount of scriptblocks you have. If your map is custom made (rather than a defualt GR map that you've just added a new mission to) then you may have problems with how you set it up. Otherwise, the only other thing I can suggest is to reduce your graphics options (lower the resolution, lower the LOD - especially the shadows) or turn off any rain or snow that you've got.

You don't mention what hardware you've got. If you only just meet the system requirements you might struggle a bit. Otherwise it must be something to do with what you've scripted...