Danny,
If your installation of FS is slow it's not likely to be due to the installed aircraft - it's much more likely to be addon scenery or planes or perhaps a computer issue, such as needing to get the latest drivers for your video or sound card. There are lots of tweaks to resolve these kind of issues on the popular FS forums such as at Avsim.com and it very much depends on what computer setup you have as what's going to be best for you.
The dangers of deleting the default aircraft are that you won't have any AI traffic any more (unless you've installed your own) and that any new aircraft, including some EHM ones, that use the default 737 sound and panel (as an example) won't have a panel or make any noise any more... I'd certainly try other options first if I was you.
Hope this helps,
