This problem is (usually) a simple fix.......
What gel system are you using?
Are you using the UV lamp that comes with that system?
What training have you had (who did you train with)?
Your post pretty much answers these questions because you say you purchased a Salon Edge UV lamp and it is taking ages to cure your Shellac top coat.
Here's the problem just with your Shellac taking so long to cure, you are not using the correct lamp but I am only saying this if you are using legit CND Shellac and the reason why is, Shellac can (and should) only be cured in the CND UV lamp if you wish to get a full correct cure, you will not achieve a full correct cure in any other UV lamp.
So the bottom line, the simple problem is, you should always use the UV lamp that either comes with the system you are using or a UV lam that your gel system recommends you use, you should not mix and match your gel brands to just any old UV lamp otherwise you can never guarantee a full correct cure and this will only lead to problems down the track, especially for your clients nails/surrounding skin.
Now, if you are using a knock off (from FleaBay for example) Shellac for example and then you are using this UV lamp you bought, then you are never going to get things right and could really do some serious damage to yourself and any clients you may get/have and I'm sorry to say this but you can also not promote yourself or your services as a professional service, as none of the combination I have just mentioned, comes under being a professional.
What I suggest you do is, if you have had not training with a reputable brand/company, then go get some training with such companies and if you have had training already, then I suggest you go for some more and again with a good reputable company and then get yourself a good quality branded product and all the correct lamps etc that come with that good reputable brand and then I will guarantee you that you will be on your way to a great professional career as a nail tech.