It will not degrade the performance it just reduces the total length you are allowed. You can run 100 meters of ethernet cable but that assumes a single run of cable. Every time you put some kind of splice in it you increase the resistance and reduce the distance but it not going to be reduced by very much. Then again I have not seen anyone test what happens is you put in 100 splices. I suspect you could use a coupler and 2 50 meter cables and it would still be fine.
Pretty much ethernet will run at 1gbit/sec or it will fail.
Cost wise the couplers are not generally your best choice. They cost almost as much as some cables.