I tried the Electro Life pwr window kit in my Challenger and it was disappointing to say the least.
The glass is so heavy they dropped like a rock and struggled to raise.
I had them directly wired to the battery so it wasn’t a voltage drop issue.
I talked to Electric life directly and they suggested I add a helper spring like the stock regulator. I could tell they had heard this problem many times before. I wasn't interested in doing that so I sent them back. It only makes sense, just like a garage door, spring winds up on way down, slowing it down, then unwinds on way up, helping it lift. Even the 40yr old factory parts have the assist spring.