If you want to gain weight fast then one of your biggest concerns is fully working your muscles to a point of momentary muscular failure.
The best way to ensure this is to slow down your repetition speed and work the muscle thoroughly, minimizing momentum.
There are two good reasons why you want to do this.
The first reason is that slowing down your repetition speed will force you to reduce the weights you use and help you to learn how to control your muscles safely.
Many long-term injuries occur by people "heaving" up weights that they could not otherwise control.
This ballistic style of lifting can lead to tendon and ligament damage, as many longtime weight trainees will privately admit.
Slowing your repetition speed will increase safety because you will then have the ability to execute flawless lifting form.
Gaining weight fast is a process where you need to be in the gym on a consistent basis; and focusing on safety will help you be able to perform at a high level in the gym consistently.
The second reason you should slow down your repetition speed is that the muscles grow under tension.
For example, if you were to perform a set of 8 repetitions on the lat pulldown machine, consider these two choices:

1. 3 sets of 8 repetitions where you would pull the bar down in 1 second, maybe have a slight pause, and let the bar raise in 1 second.
   3 sets of 8 repetitions (each repetition lasting no more than 3 seconds in total) would equal a MAXIMUM of 72 seconds.
   
2. 1 set of 8 repetitions where you would pull the bar down in 3-4 seconds, pause for 2 seconds in the fully-contracted position, and raise the bar in 4-5 seconds.
   In this case, 1 set of 8 repetitions (each repetition lasting at least 9 seconds, if not longer) would equal 72 seconds (or more).
   

The problem with the faster repetition speed example is that you used momentum and other forces, so the amount of time your muscles would be under tension most likely would be significantly less than 72 seconds.
In the slower repetition speed example, your muscles would be under tension for at least 72 seconds, and you would minimize the amount of momentum and other forces.
All of that time under tension would help your lift be more productive toward your ultimate goal: building muscle and gaining weight fast.
If you have any joint discomfort or injury, please talk with your doctor about ANY weight training regimen.
Then talk with your doctor and determine if he or she agrees that slowing down your repetition speeds (even if you have to lower the weights initially) would be more productive and safer for your muscle-gaining pursuits.