What version(s) of Android are you testing on? Are you using the Canvas tag?
There is a known issue on 4.4 when using the new Chromium-backed web view that the Canvas tag is no longer hardware accelerated, and as such, games that use it will experience severe slowdowns. Last I knew, there was no fix.
Terribly frustrating -- 4.3's webview had its problems with the canvas tag as well, but at least it was accelerated. Now 4.4's web view fixes those particular problems (ghosting, etc.), but now loses the acceleration. *sigh*
Short of a fix being available and easy to implement, atm, HTML5 games using canvas are DOA on Android 4.4. Very, very frustrating.