I don't think SpiderMonkey/Gecko are the villains here. Specs are in order to reach a price point.
Profit margins on a $25 device are zilch, so manufacturers need to cut corners by releasing products unsuited to a 2015 era OS. Try running iOS 8 or Android 5 on a handset with 128MB and a Cortex A5 CPU - that's a phone released in 2014 with specs equivalent to an iPhone 1 or a Nokia 97 from last decade.
By comparison on better hardware - Performance on their reference phone, the Flame, is decent. ZTE are releasing the Open-L this month with a quad core Snapdragon 210. Hardly iPhone 6 territory but still - The previous model has been retailing for about $US77 here in Australia. When my Flame breaks I'll willingly shell out the dough...