Does negotiation end fast or slow ?

All negotiations end the same way either you have a negotiated agreement or an impasse. How long it takes you to get there is the unknown. Something complex and highly commercial would take special resources and potentially a longer time to resolve and a failure to resolve could result in litigation. Simple less complex and not so commercial issues could be resolved without any additional resouces and in a short period of time.

in my opinion,

negotiation ends or extends based on who you are negotiating with and based on your needs and the other party's need, for example, if it is between buyer and seller for car or real estate

if the seller needs fast cash he would accept your deal 

but if the buyer is a company who has many sources for its supply chain 

may be your negotiation will extend to convince your client of other things that other suppliers do not have 

so negotiation speed depends on:

1) situation

2) need

3) market and environment

4) any other factor directly involved in the deal between you and your client

I hope this answer will help