I see the issue - it looks like it could be a "～って(いうのは)いいぜ" kind of construction. But in that case the preceding bit wouldn't be te form, it would be "動きやすいってい

I see the issue - it looks like it could be a "～って(いうのは)いいぜ" kind of construction. But in that case the preceding bit wouldn't be te form, it would be "動きやすいっていいぜ" or whatever. Rather, the っ here is just an affectation that makes the speech sound a bit childish. What's being said grammatically is just 「動きやすくていいぜ」.