Why does it lag on phone when it doesnt used to

Well, i have been using construct 2 over the span of almost 1 year and i am a proud c2 user. after some time, i bought the product because i wanted to develop it on phone and sell it. after r80, the game lags alot and on r74, it is full speed(as in, no lag). what is the problem here?

What phone are you using and what OS version is it running? Can you provide some more details such as the specific framerates and the .capx you're using?

i tested on 4 devices. an ipad 2 and the framerate was about 5, iphone 4(the framerate was also 4), galaxy s2 (its 6fps) and an xperia play (3fps). the file has 6 image in a span of 480,3000(scroll game)

We've done nothing but optimise C2 over the past few releases, so I would expect the performance would be the same or a little worse in r74 and earlier. However usually the browser isn't fast enough anyway, have you tried CocoonJS for Android in the latest beta?