What do you say about speed?

Question

Does the animation make the browser lag or anything else in the context of slowing the device.

Answer 1

It depends from device to device

Answer 2

Yes, it can... but browsers generally interrupts JS script that becomes too busy, and ask to user if he want let continue or break the execution of the probably problematic script.

On the other hand, you can have an enough busy script wich will reduce global performaces, without becoming enough busy for the previous behaviour. Then, it is fully dependant of available memory and processor ressources on client device, and of requierement of the script(s): performing lot of complex calculs to perform a 3D animation will expect more ressources than just moving one box on 2D space...

To improve your JS by saving some ressources, make all that you can from your animations using Css3: the render engine run independently of JS scripts, and would be always smoother, as not dependant of presence of activated JS in user browser ;)