Why is my iPhone 4 slow sometimes?

Is straighttalk the reason for my iPhone 4 being slow?

For starters, I play respawnables which is a mmorpg or a rpg which ever, on my iPhone 4. And when I try to play on multiplayer, it says I can't connect to server. My speeds is fast when not throttled. 3mbps. And I'm on straighttalk. Still locked to AT&T. When I do play multiplayer, its when using Wi-Fi. Maybe 2 times I've been able to play multiplayer on my phone on it's on but still had connection problems. I thought it's because my phone uses 3G but my uncles phone uses 3G, an android, and his phone can play respawnables multiplayer very smooth. And it's on sprints prepaid. It's a Motorola Moto G. I'm starting to think it's because of straighttalk. And because my iPhone 4 is old. But how fast my phone loads data, I don't think so. I've been wanting to get a new iPhone 5s because of this. Not just that though. There's other problems with this It also loads some web pages slow sometimes or not completely right.

Because of low RAM.

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