Any large download could produce some general system slowing and so reduce browsing speed - but various factors will come into play. Speed of connection is one and if slow then probably browser might behave slower during download period - you haven't supplied any info on that.
Then your system - how much speed, caching, RAM etc ... all factors affecting how much spare processing ability you have when busy. I wouldn't expect to see too much CPU use needed during this kinda thing but for sure your internet connection will have quite some influence.
If you want an e-book - and say it's 3Mb PDF - probably best to initiate the download and leave it be for the duration - then resume browsing. Save/burn to a CD will be best done secondary to download IMO.